The RHI Express has pulled into the station for the last time... well, almost.

It'll be back in December for three days of closing statements but without the full cast of characters we've come to know so well over the past year.

As for the whodunnit, that'll have to wait until Sir Patrick Coghlin issues his final report and that's yet to appear on the timetable.

Heartfelt thanks to everyone who's followed our coverage - we hope you've enjoyed the year-long excursion and trust we showed you as many of the sights along the way as possible!

Jayne McCormack
BBC News NI politics reporter

A former DUP adviser was alleged to have said "we could fill our boots" with money from the RHI scheme because he believed it was coming from the Treasury, not from Northern Ireland's own budget.

The claim was made about Dr Andrew Crawford by senior civil servant Dr Andrew McCormick on the last of the inquiry's 111 public evidence hearings.

Dr McCormick said the remark led him to believe that Dr Crawford had misunderstood how the RHI was funded.

But he added evidence uncovered by the inquiry shows that Arlene Foster's former aide knew about the potential for the scheme to be abused and "didn't tell" civil servants.

While this is the conclusion of the public evidence hearings it is "by no means the end" of the RHI Inquiry, says senior counsel David Scoffield QC as he offers some closing remarks.

Instead, this is "merely another staging post" on the road to the final report by Sir Patrick Coghlin and more evidence is to be gathered in writing and will be posted on the inquiry's website.

Referring to his colleague Joseph Aiken's fondness for railway references, he says: "I consciously eschew the opportunity to employ a train-related metaphor at this point!"

Image source, RHI Inquiry

And he reminds everyone that barristers for the core participants in the inquiry will give their closing statements next month.

"For those who may be suffering withdrawal symptoms after today there are those sessions in mid-December to look forward to!"

Mr Scoffield thanks those who have supported the inquiry team in its work and the staff at Parliament Buildings who have contributed to the smooth running of the hearings.

And there's a final thanks to the panel of Sir Patrick Coghlin, Dame Una O'Brien and Dr Keith MacLean for "your patience, your interest, your good humour... and for asking many pertinent questions that didn't occur to us!"

Sir Malcolm McKibbin was "shocked" by former economy minister Simon Hamilton's admission this week that his adviser anonymously leaked emails to his department's top civil servant in a bid to relieve pressure on the DUP.

The emails were sent at the height of the RHI storm in December 2016 and Sir Malcolm says he can't get his head around it given that Dr Andrew McCormick, the economy permanent secretary, was "sitting five yards away".

Image source, Getty Images

He's also surprised about the "quality and comprehensiveness" of advice that civil servants gave to ministers about the RHI scheme.

Speaking from a civil service point of view, he says: "Our job is to provide objective, evidence-based advice that is comprehensive and informs the minister."

Slashing the cost of the RHI scheme in winter 2016-17 would've been done "much more smoothly" if the same political party had held the ministerial roles in the finance and economy departments, says Sir Malcolm McKibbin.

Instead, Sinn Féin held the finance brief and the DUP had responsibility for economy matters.

The tension between the parties "made this all the more difficult and potentially much more expensive", according to the retired civil service chief.

Image source, RHI Inquiry

Inquiry panellist Dame Una O'Brien asks if there can be a different way of handling such tensions to ensure they don't appear again.

"It was a factor - let's not beat about the bush," says Sir Malcolm.

"If we get an executive coming back and people behaving the same way as they did in the last mandate the outcome is likely to be somewhat similar to that which we are in now."

There was good cooperation between Arlene Foster and Martin McGuinness on finding a way to cut to cost of the RHI scheme in winter 2016-17, says Sir Malcolm McKibbin.

That's a sharp contrast with how DUP economy minister Simon Hamilton and Sinn Féin finance minister appear to have been handling things - the inquiry heard from both men on Tuesday.

Image source, Pacemaker

David Sterling, the interim civil service boss who was working in the finance department at the time, said yesterday that "tension" between the parties affected the work to sort out the RHI mess.

But Sir Malcolm says things were different in the Executive Office: "Certainly when I was in the Castle, the vibes there were that: 'We are going to deal with this scheme.'"

He admits, however, that it was "going closer to the wire than anybody felt comfortable with".

Staff working for Sinn Féin's Martin McGuinness were provided with a whistleblower's letter that detailed allegations of how the RHI scheme had been abused, it emerges.

The letter was first given by the whistleblower to Arlene Foster in January 2016 and she passed it to Sir Malcolm McKibbin.

The inquiry sees email evidence that the civil service boss's staff subsequently sent it to the then deputy first minister's secretary and his adviser Mark Mullan.

That undermines Sinn Féin's claim that the letter was not shared with Mr McGuinness.

Mrs Foster told the inquiry that she talked to Mr McGuinness about it.

Away from the inquiry, Sinn Féin's vice-president Michelle O'Neill said last month that the claim that Mr McGuinness knew about the letter "is subject to an ongoing and separate legal action".

"Sinn Féin is confident that our position and the position of Martin McGuinness will be fully vindicated."

Sir Malcolm McKibbin is "absolutely certain" he didn't say the DUP leader Arlene Foster should not have been implicated in the RHI debacle.

At the end of January 2016, senior DUP adviser Richard Bullick told his colleagues in an email: "[Sir Malcolm] seems very concerned about spending out of control. Says will be huge audit issue and AF better not implicated."

Image source, Reuters

Asked to explain that, Sir Malcolm says he used the word "involved" instead - the word implicated has "all sorts of connotations".

He says it wasn't for the then first minister to be involved in intricate decision-making about the scheme closure because the expertise "clearly lay" with DETI.

A couple of weeks ago, Sir Malcolm met Mr Bullick, who's no longer a DUP adviser, and he says Mr Bullick told him he "wouldn't argue" with his insistence that he said involved, not implicated.

Sir Malcolm McKibbin says it's "not satisfactory" that he first became aware of the massive financial black hole caused by the RHI scheme in January 2016.

There was the potential for hundreds of millions of pounds of the Northern Ireland budget to spill down the drain over 20 years as a result of the scheme.

Image source, Getty Images

The ex-civil service boss says he should've been told the previous month when it became clear to officials in the enterprise department that "there was a serious problem".

"It would be something that I would have wished to brief the first and deputy first minster on," he says.

There were concerns about Jonathan Bell's "interactions with some of the senior players from big businesses" during trade missions to the United States, says Sir Malcolm McKibbin.

The former enterprise minister and DUP MLA was in change of the RHI scheme when it ran out of control but claims he wasn't responsible for much of what happened.

Sir Malcolm says he heard about the business concerns from officials at Invest NI, Northern Ireland's economic development agency - its representatives would've gone on trips with Mr Bell to woo big firms.

Image source, Pacemaker

Sir Malcolm says American companies "expect the minister to be fully engaged in any discussion they are having" and have a "good understanding of their business".

He also reveals that Mr Bell didn't contribute as much to Northern Ireland Executive meetings as other Stormont ministers.

As the head of the civil service, Sir Malcolm sat around the executive table and observed how ministers operated in that situation.

Questions have been raised several times at the inquiry about Mr Bell's competence as a minister and DUP chief executive Timothy Johnston said Mr Bell wasn't a suitable candidate to hold a role in the Stormont executive.

Sir Malcolm McKibbin occasionally felt "uncomfortable" about the role played at Stormont by senior Sinn Féin adviser Aidan McAteer.

Mr McAteer is a former prisoner and was barred from being an official ministerial adviser as a result of legislation passed in 2013 that prevented anyone with a serious criminal conviction from holding such a role.

Image source, PA

He worked at Stormont alongside the then deputy first minister Martin McGuinness (above) and Sir Malcolm says he saw Mr McAteer coordinate Sinn Féin work and handle big issues and political talks.

Sir Patrick Coghlin said on Tuesday that Sinn Féin was in "deliberate breach" of Stormont rules because people paid by parties are not to direct civil servants.

He says now he's concerned about that because Mr McAteer was directing Sinn Féin advisers who were in turn issuing instructions to officials.

Evidence the inquiry has seen suggests that some ministerial advisers at Stormont had much more influence than others and performed roles far beyond what's in the job description.

One former DUP adviser Tim Cairns told the inquiry that everyone within the party knew that Timothy Johnston was "at the top of the tree".

The inquiry also heard Sinn Féin's Máirtín Ó Muilleoir say this week that one Sinn Féin backroom figure - Aidan McAteer, who was not an official adviser and was paid by his party rather than from public funds - carried "more authority" than others.

Image source, Getty Images

Sir Malcolm says he was aware that DUP MLAs listened to Mr Johnston when they were discussing political strategy and tactics but he didn't have any particular concerns about his role.

As for Mr McAteer, Sir Malcolm says there was a "certain amount of central control exercised by party headquarters" and that's "not uncommon".

"The [adviser] role, whenever you read through their job spec, it is a bit grey in areas," he says.

But he says there "are issues around how [advisers] are operating and how they should operate in the future" and it'll be a key issue that the civil service will have to address.

Stormont's ministerial advisers and their role is a subject that has vexed the inquiry, particularly in relation to the appointment of DUP aides.

The party's chief executive and former senior adviser Timothy Johnston (below) admitted to the inquiry that it had drifted away from the code of practice for appointments but claimed the party was "trying to mirror" what other parties were doing.

Image source, Press Eye

The code states that advisers are to be appointed by ministers and several candidates must be considered but it appears that was largely circumvented and Sir Patrick Coghlin said there was a "very real concern" that the proper procedure was being "camouflaged".

Sir Malcolm says he had no evidence that the DUP was not adhering to the code but now that he sees what was going on he says: "I'm surprised - I'm disappointed."

There was a "very Northern Irish cold war" between the DUP and Sinn Féin over the RHI fiasco in winter 2016-17, suggests inquiry counsel Donal Lunny.

He points to the battling between the DUP's Simon Hamilton and Sinn Féin's Máirtín Ó Muilleoir - then the economy and finance ministers respectively - who were at odds about how to slash the scheme's costs.

Image source, RHI Inquiry

Sir Patrick Coghlin described them on Tuesday as "alley cats... fighting" while public money was being wasted.

Asked if the RHI issue should've been "grappled with" at that time by the then first and deputy first ministers instead, Sir Malcolm McKibbin says it was left with the economy department because it "needed a technical solution" that could only come from its officials.

He says that technical fix needed to be established for the scheme itself before a political solution could be found to deal with the fallout between the two parties.

During the talks to restore devolution after the Northern Ireland Assembly election in March 2017, there was support among the political parties for big changes to governance at Stormont, reveals Sir Malcolm McKibbin.

Some of the points that had been agreed were changes to the code of practice for ministerial advisers and an extension of the remit for the assembly's standards commissioner.

Image source, RHI Inquiry

It "reflects well on our politicians" that they "realised things weren't working well", says the former civil service boss.

"It had been recognised... that things needed to change, things needed to improve."

But he stresses that "nothing is agreed until everything is agreed" and the widely-supported changes haven't come into effect because the parties have yet to do a deal on restoring devolution.

There were "annual political crisis negotiations" at Stormont, says Sir Malcolm McKibbin.

"They took up months - months - and there was a huge number of hours put into those."

He was closely involved in those talks even though it "wasn't part of the job spec, let me tell you", he says with a smile.

Image source, RHI Inquiry

Sir Patrick Coghlin picks up on the reference to those negotiations being an "annual" event: "Is that a figure of speech or was there actually annual crisis negotiations?"

It most definitely was, says the retired official, and he lists the various fallouts and talks processes in 2013, 2014, 2015 and 2017.

"So it's a perfectly reasonable use of the adjective!" laughs Sir Patrick.

As the head of the Northern Ireland Civil Service, Sir Malcolm McKibbin was the chief civil servant in the Office of the First and deputy First Minister (OFMDFM), now known as the Executive Office.

Asked about the nature of the work that was carried out in Stormont's top department, he emphasises the unique nature of the power-sharing executive system and the regular crises that threatened to collapse it.

For instance, there were four ministers and eight ministerial advisers working at Stormont Castle (above) - the OFMDFM HQ - compared with 12 advisers in the whole of the Welsh government.

Every decision had to be taken jointly by the first and deputy first ministers - one DUP, one Sinn Féin - and in reality that was done through negotiation by the advisers.

"The atmosphere within Stormont Castle was highly politically charged," he says.

Now retired, Sir Malcolm McKibbin was Northern Ireland's most senior civil servant from 2011 until June last year, having started working in the service in 1978.

His involvement in the RHI scheme is fairly limited, mainly centering on the events leading to the closure of the initiative in February 2017.

Image source, RHI Inquiry

His successor as the head of the Northern Ireland Civil Service was David Sterling, who gave evidence to the inquiry yesterday.

You can read Sir Malcolm McKibbin's witness statement to the inquiry here, external.

Inquiry barrister Donal Lunny is leading the questions.

Becoming emotional at the end of his lengthy evidence to the inquiry, Dr Andrew McCormick says the RHI disaster has been "incredibly destructive" to politics and governance in Northern Ireland.

It's also "done me personal damage", he admits.

Image source, RHI Inquiry

He's a "passionate believer" in the political institutions at Stormont but they "will only work" if trust is rebuilt between the DUP and Sinn Féin.

"They understandably don't trust each other - they're poles apart in political aspiration," he says.

Building a "stable way of working" at Stormont is the big task and there's a "long way" to go to achieve that.