Mr Aiken suggests, tongue-in-cheek, that he's brought today's evidence session in "under-budget" - a good half-hour ahead of the scheduled finishing time!

Sir Patrick winds things up and apologises to Mr Brennan for dragging him back in for additional questioning.

Image source, Press Eye

We'll be back tomorrow at 10:00 GMT with the usual inquiry cast, plus another key civil servant Fiona Hepper, who was head of DETI's energy division - she faces an entire day's questioning so join us then.

Have a great evening!

According to Mr Aiken, all of the statements that the inquiry has examined so far from finance officials appear to say: "We didn't do anything wrong; we don't think we should have done anything differently; we see this as a problem that was over there in DETI."

Image source, RHI Inquiry

He asks if that is the position of the finance department.

Mr Brennan says it's possible for everyone to be doing their job properly but "cracks do appear".

Confusion among Stormont officials right at the outset of the set-up of the scheme "set the template for what people then had to sort out" when the scheme's fatal flaws emerged in 2015, says Dame Una.

Image source, RHI Inquiry

Issues that emerged later in the scheme's lifetime were "rooted" in that misunderstandings at the early stages.

"I think that would be a fair assessment," accepts Mr Brennan.

The RHI scheme became a "profound threat" to Northern Ireland's expenditure and the implications of overspending were "completely misunderstood, even in the Department of Finance", observes inquiry panel member Dame Una O'Brien.

Mr Brennan agrees with her view, saying that there was no awareness of the extent to which "what seemed like a small scheme could critically undermine" Norther Ireland's block grant from Westminster.

He says misunderstanding of the funding arrangement and inexperience of running such a scheme led to a "cocktail for a perfect storm".

"Various things [went] wrong, but when you bring them together it created the horror that we've seen."

The absence "clear, explicit" instructions from the Treasury about the funding plan for the RHI scheme in Northern Ireland led Stormont officials to draw "inferences and conclusions" about how it would work, says Mr Brennan.

The £25m originally set aside for the scheme over four years was a "minuscule" amount in terms of the overall amount of money the Treasury deals with, suggests Mr Aiken, and that may have meant it didn't received a sharp focus by officials.

Mr Brennan agrees, saying that the figure is small not just in Treasury terms but also in the picture of Northern Ireland's overall £80bn budget at the time.

Mr Aiken puts it to Mr Brennan that the email from Treasury official Mr Parker had no standing regarding the Northern Ireland RHI scheme.

"Yes, the Jon Parker email sets out how the Treasury are going to deal with DECC (the Whitehall department that operated the GB RHI scheme)," he replies.

Sir Patrick Coghlin asks what Mr Brennan understood of the Northern Ireland scheme funding arrangement was with DETI.

Image source, RHI Inquiry

He says he understood that if there was an overrun on the £25m budget there would be a penalty to DETI's budget the following year of 5%.

"That's the Parker email," says Sir Patrick, "I thought 30 seconds ago you said it didn't apply to Northern Ireland.

Mr Brennan says DFP never received a formal letter from Treasury to say that's how it would actually work.

There appears to have been a basic misunderstanding among Stormont civil servants about the repercussions of an overspend in the RHI scheme, suggests Dr MacLean.

He says there is a "whole series of people" who, up until 2015-16 - the year in which the scheme accumulated much of its vast overspend - believed that the only problem if it went overbudget would be that there would be a 5% penalty to pay to the Treasury.

Image source, Getty Images

He suggests that their view might've been: "It's not that much, we'll cope with it."

But he says there are other who understood the consequences correctly - that is, any overspend would have to come out of DETI's budget and there would also be the 5% penalty.

Mr Brennan accepts that he didn't realise the full impact of overspending until the 2015-16 financial year.

Inquiry panel member Dr Keith MacLean has been looking at some of the spreadsheets concerning the RHI scheme and other projects that were funded in a similar way.

Image source, RHI Inquiry

"It looks like there are other things that are being [carried over] and moved around and in that bigger picture the £1.8m is actually relatively small," he says.

"For me, at the moment it's not screaming out that this is really unusual because that's a small line amongst a dozen others, and its a tiny sum in comparison with the total," Dr MacLean adds.

Back to the chamber then, and to the carrying over of unspent funding in the first financial year - 2011-12 - of the RHI scheme.

A total of £2m had been set aside for DETI to used for the initiative in its first year but a low number of applications meant the vast majority of that money was not spent.

Typically, that unspent funding would then be lost to the department - in other words, it would have to hand it back to the finance department - but in this case it applied for the money to be carried over to the next year.

That request was approved, and the budget for the next financial year, which had been due to be £4m, was increased to £5.8m.

Conor Macauley
BBC News NI Agriculture and Environment Correspondent

Away from the Senate chamber, it's emerged that another whistleblower raised concerns with officials running the RHI scheme about allegations of abuse.

The renewables company Solmatrix wrote to DETI to highlight problems about "unscrupulous beneficiaries", according to documents published by the inquiry.

The inquiry had heard that only Janette O'Hagan, who ran an energy business, had flagged such issues, but her concerns were ignored.

Stormont's Department for the Economy - formerly DETI - says it has also examined other material that has added to its understanding of what went wrong.

It had seen new evidence that has caused concern that there had been a "conspiracy of silence" about the scheme's problems.

"Indeed, some of the evidence appears to go beyond even that description," it adds.

Mike Brennan, the the head of the Department of Finance's central expenditure division, has returned to finish giving his evidence after the session had had before the inquiry last week.

Inquiry counsel Mr Aiken reassures him, and the rest of us, that his questioning will not take all afternoon.

Image source, RHI Inquiry

The session begins with questions on a subject touched on just before lunch.

It emerged that money not used in the first year of the RHI scheme first financial year was carried over to the following 12 months.

It had originally been understood by DETI officials at the time that any unspent money would have to be handed back to the Treasury.

Mr Aiken tells the panel that he's not finished with his questioning of Ms Brankin and she'll have to come back another day to finish giving her evidence.

Image source, Getty Images

That's hardly surprising since almost every single session appears to be overrunning its time slot.

We'll be back at 14:00 to hear from Department of Finance official Mike Brennan - join us then.

Grant and subsidy schemes in Great Britain are "normally copied to the letter of the law" when they are implemented in Norther Ireland, says Ms Brankin.

We know now that the Northern Ireland RHI scheme was missing crucial cost controls that were added to the Great Britain scheme, and it was those missing features that led to the scheme going vastly over its budget.

Image source, Getty Images

Ms Brankin says there would not normally be a deviation from GB schemes because they are the "benchmark", and she adds: "If you were deviating from that you would need a very good reason."

She adds that her message to DETI had been that there needed to be budget controls in place in the RHI scheme.

Ms Brankin says her concern about the budget for the RHI scheme was two-fold.

She didn't want an overspend, because that would result in a penalty, but she understood that if the budget was not spent the remainder would have to be given back.

Image source, RHI Inquiry

She says she made that point in an email to Alison Clydesdale, one of DETI's energy officials, who was working on the scheme.

She says she met Ms Clydesdale to beforehand to discuss the funding arrangement in detail because she saw it as significant: "I tried to make the email as clear as possible".

Mr Aiken points out that the inquiry has recently discovered that what Ms Brankin had feared in the case of an underspend did not actually happen - the money not used in the scheme's first year was carried over to the next year.

The inquiry has found no documentary evidence of the conversation Ms Brankin had with Stuart Stevenson, says Mr Aiken.

She says she would have made notes of their conversation but she cannot remember as it happened too long ago.

Image source, RHI Inquiry

Inquiry chair Sir Patrick Coghlin asks whether she kept her notebooks after her retirement in 2016.

"I have 30 years service so I wouldn't have kept them all," Ms Brankin says.

Ms Brankin says the relationship between DETI and DFP officials were good but it was "quite an informal" one and she adds: "I don't think you're going to find it documented in many cases."

Mr Aiken asks Ms Brankin what her immediate reaction to Mr Parker's email was.

She says she had no experience of the arrangement that was outlined and she "would have needed to seek advice" - as a result, she sought clarification from Department of Finance and Personnel (DFP) officer Stuart Stevenson.

Image source, RHI Inquiry

Mr Stevenson, she says, advised that the £25m funding allocated to DETI for the RHI scheme over four years should be treated as a set pot of funding that shouldn't be exceeded because any overspend would fall on the department.

Typically, funding for a demand-led schemes - as the RHI was - was not considered in that way as they could be difficult to predict because the spend depends on what the uptake - the number of applications for the schemes - will be.

This morning has been heavy on technical detail, but Mr Aiken moves on to what's been a recurring piece of evidence in the inquiry so far.

It's an April 2011 email from Jon Parker, the joint head of the Treasury's energy branch, who told DETI how the RHI scheme in Great Britain worked.

He said the Treasury was not footing any overspend, and instead the Department of Energy & Climate Change (DECC), which set up the Great Britain scheme, would need to cover any overspend in future years.

Image source, PA

Those rules would then be applied in Northern Ireland when its own scheme got up and running, he said - basically, that meant the money wasn't limitless and DETI would have to repay any overspend if the scheme went beyond its set budget.

Mr Parker also said there would effectively be a penalty imposed of about 5% in the case of an overspend.

The civil servants who have been questioned at the inquiry so far about this financial arrangement have all said they had never come across one like it before.

Finance officials at DETI were working under "pressure" to meet "very tight" deadlines to get projects approved, says Ms Brankin.

Image source, Getty Images

The nature of their work meant they were often "in the building until very late at night" and also did work at home in evenings, she adds.

"You needed to put the hours in to make sure the deadlines were met."

Mr Aiken refers to evidence given by Ms Brankin in a lengthy interview to the consultancy firm PwC, which was hired to carry out an internal investigation of the RHI scheme in 2015 after its major problems emerged.

In that evidence, Ms Brankin expressed the view that DETI "shouldn't really be doing schemes like the RHI".

Image source, RHI Inquiry

"Yes, that's my view, and it still is my view, Ms Brankin says.

She says she thought the expertise to deal with matters such as grant payments instead lay in "non-departmental public bodies" such as Invest NI, "which had the experience and the experts".

Ms Brankin explains that her branch within DETI had financial responsibility over a broad range of issues, of which the RHI scheme was a small part.

She says casework notes on the scheme's early stages were not shared with her branch, but that would have been normal - instead, there was a specific casework branch that dealt with that.