Simon Hamilton's DUP adviser anonymously sent emails that detailed contact between officials and industry about the RHI scheme in 2015 to their own department's top civil servant and journalists.

John Robinson did that because the emails supported an assertion by fellow DUP adviser Dr Andrew Crawford that he hadn't caused the hugely damaging spike in applications.

Mr Hamilton admits it's "not my proudest moment" and is one of "many things that I regret" doing at the time when the DUP was under huge pressure about the RHI in the winter of 2016-17.

He thought the emails were "pretty explosive" and discussed with his adviser about how it "would be useful if it was in the public domain".

Image source, Getty Images

Inquiry panellist Dame Una O'Brien asks: "[You sent it] to your own permanent secretary anonymously? I just want to be sure I heard that properly."

She can't understand the rationale for doing that and asks how Mr Hamilton thought it would improve the situation and how it was consistent with Stormont's ministerial code.

Mr Hamilton admits it's "highly unorthodox" and says it was done in the context that the DUP was being "assailed on all sides - we're getting hit left, right and centre every day" and there wasn't "much to fire back."

Inquiry chair Sir Patrick Coghlin says Mr Hamilton and his adviser had "used the cloak of anonymity" to take pressure off the DUP.

Simon Hamilton was "absolutely determined" to publish the names of RHI scheme claimants in January 2016 because the issue had become an "Achilles" for the DUP, he says.

He tells the inquiry it was suggested that the DUP didn't want the names of claimants to be revealed because it would show that "all your mates" were benefitting from it.

Image source, RHI Inquiry

People, including politicians and the media, weren't accepting his explanation that the publication would've gone against legal advice given to the economy department, he says.

His decision to release the names in spite of huge opposition from claimants was prompted by the need to be "transparent" about an issue that was a "constant, constant thorn in our side".

The economy department's preferred cost-cutting option of paying off RHI claimants was leaked to the press in December 2016 - the resulting negative reaction effectively killed it off, Dr Andrew McCormick (below) told the inquiry.

The DUP and Sinn Féin had suspicions that each other was responsible.

Mr Hamilton says he didn't leak it and nor did his adviser John Robinson and he claims he can't think of a reason why senior officials might have let it slip.

"I was raging that it had happened," he says.

He thinks Sinn Féin was responsible and "thought that they were trying to undermine" attempts to find a solution to the RHI problem.

Simon Hamilton says the option he preferred to cut the cost of the RHI scheme "did the job" as a "tourniquet" to "stop a lot of the haemorrhaging" of public money.

It was intended as a short-term, one-year-only measure that would have an immediate impact on mitigating the financial damage and would buy officials time to work up a permanent fix.

It brought every RHI claimant - regardless of when they'd signed up - on to the reduced subsidies that were introduced in November 2015.

Image source, RHI Inquiry

The plan was passed by MLAs in January 2016 and while Mr Hamilton admits it wasn't perfect he defends it.

"In my view, in the situation we found oursevles in, it was far more preferable to bring something forward than... nothing forward," he says.

"In the preceding financial year the cost to the executive was £27m, I think - it reduced that down to £3m."

Simon Hamilton was "agnostic" about the method that would be used to slash the cost of the RHI scheme - he says he just wanted something that would work and would be "legally robust".

"If it achieved its objectives, if it brought costs within controls and did away with perverse incentives it was something that I would've backed," he says.

One option was to buy out claimants by giving them a sum of money that would cover some of their costs and provide them with a 12% rate of return.

It was described by the economy department's top civil servant Dr Andrew McCormick as "much cleaner" than other options and "strongly favoured" by civil service lawyers - he became "fixated" with it, according to Mr Hamilton.

But the former minister had "concern" that it would mean taking a huge sum of money from Stormont's capital budget and criticism would've mounted if that figure had crept up beyond the initial projection.

"From a political aspect, I was kind of mindful that even at the £50m level people would mount an argument that that's five new schools that could've been built that's paying for the compensation on this".

Finding a way to slash the cost of the RHI scheme was largely left to the Department for the Economy but Simon Hamilton says "pressure" could've been lifted if it had been dealt with by the Stormont executive.

He tells the inquiry that the executive did deal with some problems "centrally" but they were often "minuscule" in comparison with the RHI overspend.

Image source, PA

"This seems so obvious and you wonder why it wasn't done... if there was a big, big lesson about this that's something we ought to have done."

As a result, the economy department didn't get the necessary "kick in the backside" from the then first minister Arlene Foster and then deputy first minister Martin McGuinness to get the RHI problem sorted.

A BBC Spotlight programme that brought the RHI fiasco into full public view in December 2016 caused the economy department to give "greater focus" to trying to cut the scheme's cost, says Simon Hamilton.

Having binned the original proposals to solve the budget problem, serious attempts to address the financial threat only accelerated that month.

But it's not fair to imply there was no focus on it before the Spotlight programme was broadcast, adds the former minister.

He says the DUP believed the way to "put a lid on it" was to find a solution rapidly.

Conor Macauley
BBC News NI environment correspondent

Simon Hamilton's "heart sank a little" when he realised the plan to cut the RHI scheme's cost was drawn up by officials who had been running the initiative when it "had gone off the rails" the previous year.

Inquiry panellist Dr Keith MacLean questions whether it was wise to allow the same civil servants who had failed to spot the scheme's key flaws to then try to come up with a rescue plan.

"They hadn't had the insight to understand the problems themselves so how on earth were going to find the solutions?" he asks.

Not only that but the same officials had freely given the energy and agri-food industries advance notice in summer 2015 of cuts to the RHI's subsidies, in turn causing a spike in applications that burst the budget.

Simon Hamilton accepts that Dr MacLean's point is fair and tells him that when the proposals landed on his desk in October 2016 he was "concerned".

Simon Hamilton felt "dismay" when he read the plan that officials presented to him at the end of October 2016 to slash the cost of the RHI scheme because it didn't contain any options for how to achieve that.

The "straw that broke the camel's back" was the officials' "significantly wrong-headed and also dangerous" intent to consult with organisations, including the Ulster Farmers' Union and the poultry production giant Moy Park.

Image source, Getty Images

He believed they would've "not been best pleased" with whatever the plan would be and felt that would create a "groundswell of opposition that would make it difficult" to get any cost cuts approved by MLAs in the Northern Ireland Assembly.

The proposals were "so fundamentally flawed" that they had to be started all over again - he told officials he was "not happy with this" and their plan was withdrawn.

Simon Hamilton could've exerted more authority on his economy department officials in coming up with a plan to sort out the RHI scheme's problems, suggests Dame Una O'Brien.

The inquiry panellist and former top Whitehall civil servant reminds him that he was an experienced Stormont minister and tells him he could've asked for regular meetings with staff and set them a deadline for a plan to land on his desk.

Image source, RHI Inquiry

Mr Hamilton "wouldn't have had the need to do that previously", even in the Department of Health with its huge brief, he says.

But he accepts that he "could've... laid down my authority" more than he did.

Sir Patrick Coghlin says the fundamental problem with the RHI scheme was its overspend and the potential effect on the Northern Ireland Executive's budget but no-one was talking about that.

He notes that Simon Hamilton refers in his written testimony to officials being "delusional and shell-shocked".

Image source, RHI Inquiry

The inquiry chair asks if that applied to all of the officials from the permanent secretary right down to junior civil servants.

Mr Hamilton says people were doing work on drawing up the RHI solutions "but it wasn't to the heart of the matter".

Sir Patrick says the "simple key... is leadership" and Mr Hamilton reiterates his regret at "not pressing more than I did".

Simon Hamilton regrets that he "didn't push and push and push more than I did" to keep officials focused on finding solutions for the RHI scheme's overspend.

He knew it was "complex" work and in that sense he "wasn't unduly alarmed" that it was taking time to find fixes.

But he became "agitated" later in summer 2016 that nothing had been presented to him and the "sad truth" is that when he finally did receive something in late-October it was "shoddy".

Image source, Getty Images

There was a "number of distractions" that got in the way of officials finding a solution to the problems, he adds.

He felt they were "getting bogged down" in other work, including assisting the Northern Ireland Audit Office with its investigation into the scheme, which resulted in a savage report in June 2016, external.

He acknowledges that was "important" work but believes it took his officials' attention away from what he wanted to achieve.

When Simon Hamilton arrived in post at the economy department there was evidence of a lot of work going on in terms of reviews of the RHI scheme.

But he wanted to prioritise work on finding a solution to the overspend and to prepare work on investigating allegations of fraud and abuse of the initiative.

In particular, he wanted work to begin on a business case for the inspection of biomass boilers registered on the scheme.

"There wasn't a huge amount of evidence at that stage that there was work undergoing in terms of a solution and pressing forward with inspections," he tells the inquiry.

Cutting the cost of the RHI scheme was a "priority" for Simon Hamilton when he started his role as Stormont's economy minister, he says.

At the time, the department was faced with a potential overspend on the RHI of several hundred million pounds.

He says he made the need for priority action clear in his first meeting with the Department for the Economy's permanent secretary - the top civil servant - Dr Andrew McCormick and was assured it was top of the agenda for officials too.

But in his first-day brief in his new role, he was told the RHI was "very successful" in terms of the number of applicants and its effect in increasing heat production from renewable sources.

Image source, RHI Inquiry

In his written evidence he says that demonstrates a "startling lack of awareness" on the part of officials.

He tells the inquiry that use of the word 'successful' began to "litter" later documents about the RHI and he had to tell officials "it is never to be referred to as 'successful' again".

He "didn't think that many people would agree" it was a success story given the massive cost to the public purse and the allegations that some claimants had abused the scheme purely to make money.

"It felt to me almost as if... the operation was successful but the patient died."

Simon Hamilton says he first became aware of problems with the RHI scheme in January or February 2016 when the scramble was on to get it shut down.

"I was aware of the issue, particularly around budgetary overspend," he says.

He was health minister at the time, which was a taxing role in its own right, and he "can't recall" the RHI being discussed at meetings of the Northern Ireland Executive.

He thinks he was offered the economy portfolio in May 2016 because he had worked in the Department of Finance and because of his length of service in other ministerial roles.

BBC News NI

Two former Stormont ministers have clashed over their interpretations of events surrounding attempts to cut the cost of the RHI scheme.

Image source, BBC/Pacemaker

In his written evidence, the DUP's Simon Hamilton accuses Sinn Féin's Máirtín Ó Muilleoir of "interfering and stepping beyond his brief".

But Mr Ó Muilleoir claims the approach adopted by Mr Hamilton "hindered development" of a solution to the RHI problems.

The DUP MLA Simon Hamilton takes the oath and begins his evidence, with the questions coming from the inquiry's senior counsel David Scoffield QC.

Mr Hamilton has been a Northern Ireland Assembly member for Strangford since 2007, having qualified as an accountant before his move into politics, and he's been one of the DUP's most senior figures for several years.

Image source, RHI Inquiry

He's held three of the biggest ministerial roles at Stormont - finance, health and finally economy in May 2016.

It's his time as economy minister that will be under the mircoscope today because he was overseeing the department's attempts to cut the cost of the RHI after its closure at the end February 2016.

Mr Hamilton has provided the inquiry with two brief statements - you can find them here, external and here, external.

Jayne McCormack
BBC News NI politics reporter

From a "total lack of communication" to the "worst ever" audit report, it's fair to say it was a bruising week for the Northern Ireland Civil Service in relation to its handling of the RHI scheme.

Read our round-up of last week's evidence at the inquiry about the cash-for-ash scandal.

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 operates.

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.