Jonathan Bell was the minister responsible for the RHI scheme in the summer of 2015, when there was a delay in dealing with a civil service recommendation for action to address its spiralling costs.

Mr Johnston says he doesn't believe "we would be where we are today" if Mr Bell had not been given the top job at DETI.

Image source, RHI Inquiry

Arlene Foster had been working at DETI for a considerable time and so had her adviser Dr Andrew Crawford, he adds.

Donal Lunny suggests that those who appointed Mr Bell in May that year could be said to bear some responsibility for what happened.

"I'm not sure that I would accept that," says Mr Johnston.

Former enterprise minister and DUP MLA Jonathan Bell was not a suitable candidate for any Stormont ministerial role, claims Timothy Johnston.

He advised the then first minister Peter Robinson not to appoint Mr Bell to head the Department of Enterprise, Trade and Investment (DETI).

Image source, Pacemaker

But Mr Bell was "very loyal" to Mr Robinson and the "premium on loyalty" was something the then DUP leader "would have been mindful of".

Mr Johnston claims that Mr Bell didn't always put in the "effort" to be a minister and enjoyed the "glamour... without the work".

He "got on very well" with Mr Bell but he hasn't seen or spoken to him since the night of his explosive interview with the BBC's Stephen Nolan about the RHI fiasco in December 2016.

It took 10 days to agree the two-page letter about Brexit that Arlene Foster and Martin McGuinness sent to the prime minister in 2016, reveals Timothy Johnston.

He says those behind the scenes at Stormont "didn't always want the public to see the elements of the sausage machine because it wasn't always pretty".

Image source, Pacemaker

The Brexit letter was sent to Theresa May in August 2016, with the then first and deputy first ministers insisting that they must be involved in negotiations about the UK's relationships with the EU and other countries.

Civil service boss David Sterling said this month that Northern Ireland's entire Brexit policy was effectively based on the 11 paragraphs it contained.

Timothy Johnston explains that to get an item of business approved by the Stormont executive "meant a compromise in another area as a trade-off".

"You get into a horse-trade exercise," he says and that was "a price that you had to pay".

Image source, RHI Inquiry

Inquiry panellist Dr Keith MacLean (above) questions whether Stormont departments were capable of dealing with something as complex as the RHI scheme.

Mr Johnston says that in the future it'll have to be asked whether the Northern Ireland Civil Service has the capacity to carry out all of the tasks it wants to and whether it's even desirable to try.

The typical "man on the street" would quickly "head back out the door" of Stormont in "great despair" if he saw how devolved government was being run, says Timothy Johnston.

He admits that in his 10 years working at the top of the political tree he "didn't always have an eye to the process and the procedure".

Image source, RHI Inquiry

Discussing the non-adherence to the rules on appointing advisers, he says "you come back to weighing up what is in the greater good".

"You ended up on many occasions getting good outcomes for Northern Ireland... and weighing that with a greater weight in the balance than the ugliness of how you got there."

After the inquiry the political parties will have to look at "how to build confidence", including addressing transparency, accountability and pay at Stormont, he adds.

It "cannot be a satisfactory situation from a democratic point of view" that the mandatory code for appointing advisers was ignored, says Sir Patrick Coghlin.

Timothy Johnston says a process was reached "that probably worked for the output of the system but didn't comply with the letter and the spirit of what had been passed" by the Northern Ireland Assembly.

Image source, AFP

He says that Northern Ireland could learn lessons from other legislatures but due to the nature of Stormont's mandatory coalition and the parties involved "it may not always be possible to be as pure as what is done in other places".

Sir Patrick isn't convinced: "You're ignoring what the assembly has passed - you're building a very powerful centralised structure."

It's therefore not surprising, he says, that advisers would look to the party for direction rather than to their minister.

There was a "great drive to centralise power" within the DUP in its appointment of ministerial advisers, suggests Sir Patrick Coghlin.

He wants to know Timothy Johnston's reasoning as to why the first minister and his top aides allocated advisers to ministers, rather than individual ministers appointing their own, as the proper process requires.

Image source, RHI Inquiry

Mr Johnston accepts that there "was an element of centralisation" and a sense of "trying to mirror" what was happening in other parties.

He also says that due to the nature of the sensitive work an adviser is required to do in negotiating with other parties, the DUP needed people with whom it had a "pre-existing relationship".

"That maybe wasn't available from people brought in [from] outside the [Stormont] system."

Letters sent by ministers to senior civil servants explaining and confirming their selection of advisers were nothing more than a "tick-box exercise", says Timothy Johnston.

The inquiry has taken more than a passing interest in the letters after ex-minister Jonathan Bell told the panel that he simply signed a prewritten one that made false claims about the appointment process.

Mr Johnston says he's never seen one of the letters.

Image source, RHI Inquiry

Sir Patrick Coghlin (above) is perplexed that Mr Johnston hasn't seen any of them, given that the inquiry has been looking at adviser appointments for several weeks.

Asked if he takes any interest "in the matters that the inquiry has been looking at", Mr Johnston says he does but he hasn't seen the letters.

"That seems extraordinary," says Sir Patrick.

The DUP's appointment of ministerial advisers has been under the microscope at the inquiry, with some eyebrow-raising evidence prompting the chair Sir Patrick Coghlin to say there was a "very real concern" that the proper procedure was being "camouflaged".

A code of practice is in place, stating that advisers are to be appointed by ministers and several candidates must be considered but it appears that was largely circumvented.

Image source, Getty Images

One former DUP adviser told the inquiry that the party exercises an "unofficial procedure" for appointing advisers - he claimed that the decisions were made by a "triumvirate" of the then leader Peter Robinson and his close advisers Timothy Johnston and Richard Bullick.

Mr Johnston tells the inquiry that it's fair to say the party has had an influence on the appointments and he and Mr Bullick were "consulted".

Inquiry barrister Donal Lunny puts it to Timothy Johnston that although he hasn't held elected office he's been "very close to the heart of the devolved government here for the best part of a decade".

"I don't think you could have been much closer," responds Mr Johnston.

He also says he had "some very direct interactions at the senior heart of government at Westminster" as a result of the Westminster pact with the Conservative Party.

Timothy Johnston's time as a DUP ministerial adviser to Arlene Foster came to an end in January last year, after which point he took on what he calls a "much more informal role within the party".

He says it was "right and proper" to stay with the DUP to advise on the Northern Ireland Assembly election in March last year and the Westminster election a few months later.

Image source, RHI Inquiry

After the DUP's success in the general election last June, Mr Johnston was a key cog in the party's negotiations with the Conservative Party that led to the confidence and supply agreement.

He was appointed as the party's first chief executive in Sepetember last year.

There are several topics that the inquiry will be asking Timothy Johnston about.

Image source, RHI Inquiry

They are:

• how DUP ministerial advisers were appointed
• Jonathan Bell's role as Stormont's enterprise minister
• the relationship between Mr Bell and his adviser Tim Cairns
• Mr Johnston's position in the DUP
• work on the RHI scheme in the summer of 2015
• the closure of the RHI scheme in February 2016

Although he shies away from the public eye, Timothy Johnston has long been regarded as the primary behind-the-scenes influence in the DUP and last year he became the party's powerful chief executive.

In the days when the party opposed David Trimble's Ulster Unionists, he was its communications director, working particularly closely with Peter Robinson.

He was involved in the negotiations around the St Andrews Agreement that led to the restoration of devolution in 2007 and was an adviser at Stormont Castle for Ian Paisley, Peter Robinson and Arlene Foster.

Image source, Press Eye

The Paisley family publicly blamed him for compiling a critical survey about Dr Paisley's leadership before the party founder resigned - it was a claim Mr Johnston said was regrettable and inaccurate.

He is one of the DUP advisers that Jonathan Bell claimed would not allow the RHI scheme to close in early-autumn 2015 but he has rejected that allegation.

As an adviser to Arlene Foster, he was paid the maximum salary allowed for the job - £91,809.

Having sworn to tell the truth, Timothy Johnston takes his seat, ready to face questions from the inquiry's junior counsel Donal Lunny.

Image source, RHI Inquiry

The DUP's chief executive has provided two witness statements to the inquiry - you'll find them here, external and here, external.

Mr Johnston, who's 40, is told he's been called before the inquiry to answer questions on his role as an adviser to first ministers Peter Robinson and Arlene Foster in 2015 and 2016.

Jayne McCormack
BBC News NI politics reporter

Two anonymous complaints were made that a DUP adviser was fraudulently claiming money through the RHI scheme, the inquiry heard.

It was claimed that Stephen Brimstone was breaking the rules by using a biomass boiler to "heat his mansion house at the taxpayer's expense".

Image source, RHI Inquiry

He had installed the boiler in August 2015 - he was an adviser to Peter Robinson and Arlene Foster during their spells as Stormont's first minister.

But an investigation was carried out and Mr Brimstone was found to be complying with the scheme's rules.

BBC News NI

It is the scandal that sent Northern Ireland's devolved government up in flames and risked leaving taxpayers hundreds of millions of pounds out of pocket.

The RHI debacle has exposed serious flaws in the region's civil service and governance systems.

BBC News NI has followed every minute of the inquiry into the scandal since it started last autumn.

Read our review of some of the major revelations that have emerged so far.

BBC News NI

An independent inquiry into the RHI scandal was established in January last year by the then finance minister Máirtín Ó Muilleoir.

He ordered it in the wake of the huge public concern and what was then a major political crisis surrounding the scheme.

The RHI Inquiry began in November last year and Sir Patrick Coghlin (below), a retired Court of Appeal judge, is its chair and has been given full control over how it will operate.

Image source, Pacemaker

It is looking at:

• the design and introduction of the RHI scheme
• the scheme's initial operation, administration, promotion and supervision
• the introduction of revised subsidies and a usage cap for new scheme claimants in autumn 2015
• the scheme's closure in February 2016

For more information on the RHI Inquiry, you can read our handy Q&A.

When the scale of the overspend emerged, public and political concern rocketed.

As the minister in charge of the Stormont department that set up the RHI scheme, the DUP leader Arlene Foster faced calls to resign from her role as Northern Ireland's first minister in December 2016.

Image source, PA

She resisted, and Sinn Féin's Martin McGuinness then quit as deputy first minister in protest at the DUP's handling of what had by then become a full-blown political crisis.

That move brought about the collapse of the Northern Ireland Executive. Now, well beyond a year-and-a-half on from that, Northern Ireland remains without a devolved administration.

You can find much more detail on the RHI scheme in our need-to-know guide.

The budget of the RHI scheme ran out of control because of critical flaws in the way the initiative was set up.

Claimants could effectively earn more money the more fuel they burned because the subsidies on offer for renewable fuels were far greater than the cost of the fuels themselves.

At one point the estimate for the overspend was set at £700m if permanent cost controls weren't introduced - temporary cuts have since pulled the budget back on track for now.

Whatever the scale of the bill, it will have to be picked up by the Northern Ireland taxpayer.

The Renewable Heat Incentive scheme - or RHI for short - came to the fore of the Northern Ireland public's knowledge in late 2016.

The fallout from the scandal attached to it is still being felt in the region's politics today.

Image source, Getty Images

The scheme was set up by the Northern Ireland Executive in 2012 as a way of encouraging businesses to switch from using fossil fuels to renewable sources for generating their heat.

Those who signed up were offered financial incentives to buy new heating systems and the fuel to run them.