Tim Cairns, the former DUP adviser, has told the inquiry that he raised with concerns with Mr Stewart about potential fraud in the scheme - issues that had been brought to his notice by fellow DUP adviser Dr Andrew Crawford.

Mr Cairns said he raised the matter with the senior DETI civil servant at least 10 times.

But Mr Stewart says it was more like twice and that he was given no source for the concerns or details about what was being alleged - there was a "a general vagueness" about the reports.

Image source, RHI Inquiry

Mr Stewart had his officials check with the scheme's administrator Ofgem, which replied that there had been only four incidences of problems - all of them minor matters.

He's also highly critical of Ofgem's method of inspections, saying they were "devoid of utility" because claimants were given prior warning that they would receive a visit from auditors.

Later investigations of RHI scheme installations, carried out by by the professional services firm PwC, found that there was concern or suspected fraud in more than 50% of boilers that were examined.

The early closure of the RHI scheme was the "worst possible outcome" for DETI and for industry, says Mr Stewart.

It was shut down in February 2016 because the cost had spiralled out of control after applicants clambered to sign up, having been encouraged to do so by some within industry.

Image source, Getty Images

Mr Stewart wonders "whether on mature reflection" industry figures "thought sufficiently not only about our interests but their own" in relation to the scheme's sustainability.

It was industry's interests to "give us good, accurate, truthful, valid information" about their plans to "legitimately take advantage of the scheme", allowing the initiative to be a success and keep on running.

Civil servants clearly "had insufficient understanding of the market conditions" regarding the installation of renewable heating systems, including in areas such as poultry, Mr Stewart admits in his witness statement.

They also misunderstood the industry's ability to react quickly to market conditions, one of the reasons that caused the major spike in application to the RHI scheme in autumn 2015.

DETI officials tried to rectify some of their lack of understanding by consulting with industry figures.

Mr Scoffield asks if the department was "played" by the industry.

"I don't think we were best served by the responses that we received and I don't think industry was best served by the answers that they gave," Mr Stewart says.

Delaying the addition of cost controls to the RHI scheme by just a week would've added at least £2.6m a year over to the cost of the initiative over its 20-year lifetime.

That query made for a constituent on behalf of Mrs Foster to hold off on the assembly debate on adding the cost controls was "unusual and certainly unwelcome", says Mr Stewart.

Image source, PA

He tells the inquiry that civil servants "felt the need to dispatch it fairly quickly".

Mrs Foster's evidence is that she accepted their answer and the intervention had no delaying effect and Mr Stewart agrees with her view.

A DUP adviser asked on behalf of Arlene Foster (below) whether the Northern Ireland Assembly debate on passing the cost controls for the RHI scheme could be delayed by a week or two, the inquiry hears.

Tim Cairns posed the question after Mrs Foster had received a query from a constituent in her Fermanagh and South Tyrone constituency.

It's been claimed by senior civil servant Dr Andrew McCormick that Mr Cairns referred to a concern that "not enough businesses in Fermanagh had been able to apply" before the cost curbs would be put in place.

But he takes issue with that and denies that such wording was used.

Mr Stewart says he remembers his colleague discussing that conversation and referring to that wording - he says it was a "rather unusual phrase".

They "both took that as a not-very-subtle indication" that the query had come from Mrs Foster.

The RHI scheme's cost controls were due to come into effect at the start of November 2015 but there was a two-week delay to that, which had "very significant implications" for the initiative' overall cost.

In that time, applicants piled into the scheme to qualify for the most lucrative subsidies, contributing massively to the vast projected overspend.

Image source, Getty Images

The delay was caused by what Mr Stewart describes as "unforseen difficulties" in getting some aspects of the legislation - parts not related to cost controls - pushed through.

Mr Stewart says he "should've been more curious about" the the process and acknowledges that the legislation should've been narrowed in order to make it easier and quicker to pass.

Senior civil servants should've been "more sensitive" enough to the "extreme risk" of a delay in getting the cost controls into effect, he says, and should've "prioritised this more than was the case".

The degree of "resistance" to DETI's attempts to add cost controls to the RHI scheme was "surprising", Mr Stewart says in his witness statement.

He adds that he's never encountered a "similar phenomenon at any time during my career" as a civil servant under ministers from several political parties.

Expanding on this, he says he could "see no good reason" for the plan to be opposed and he agrees with the suggestion that there were delaying tactics on the part of DUP advisers.

"It seemed to me that we were not getting rational or reasoned counter-argument but resistance," he adds.

A meeting was held in late August 2015 in which the DETI minister approved the addition of cost controls to the RHI scheme with the added stipulation of a one-month delay in their introduction.

No record was taken of the meeting - another example of the absence of minutes that has dogged the work of the inquiry.

Image source, RHI Inquiry

Mr Stewart was on annual leave so he's limited in what he can say about the meeting but he's asked whether the lack of notes was a breach of civil service procedure.

"My understanding is that, yes, there was guidance in place that we were not following," he replies.

Mr Stewart told the DETI minister's adviser that if they wanted to introduce the proposal to increase the usage threshold for RHI scheme claimants it would require a formal direction from Mr Bell to overrule civil servants' objections to the plan on value-for-money terms.

He tells the inquiry that he tried to give Mr Cairns a sense that it was "a line you shouldn't cross - this is very serious".

Image source, RHI Inquiry

And he says that the minister and adviser would've needed an "overwhelming cause or argument" to go against the advise of senior civil servants.

He believed that was enough for Mr Cairns to take back to those within the DUP who were behind the suggestion to say that it was "something that officials thought was unacceptable".

In dismissing the proposal for increasing the usage threshold for RHI scheme claimants, Mr Mills (below0 pointed out that 3,000 hours a year would be more that was required annually to heat a poultry shed.

Many of the claimants on the RHI scheme are poultry farmers.

Mr Mills said that "suggestions of heating empty poultry sheds cannot be defended in value-for-money terms".

Image source, RHI Inquiry

Mr Stewart says he doesn't believe that DETI's energy boss was suggesting that was actually taking place but was giving a hypothetical example.

He says Mr Mills' advice was "a basis to go back quickly" to the minister's adviser to "say [the 3,000 hours proposal] is not feasible".

The witness says he now realises it also represents "a clear warning" that if that is an issue with the 3,000 hour figure "then its more of an issue with no tiering limit".

At the end of July 2015, Mr Cairns put forward a further proposed change to the planned amendments to the RHI scheme.

It was one that originated from his fellow DUP adviser Dr Crawford - the plan was to increase the threshold of hours that a claimant could run their biomass boiler for before RHI scheme payments would be reduced.

DETI's plan was to mirror the threshold in the similar RHI scheme in Great Britain of 13,14 hours but Dr Crawford wanted that to be more-than-doubled to 3,000.

The inquiry barrister says it was dismissed "very quickly and robustly" by DETI's energy boss John Mills, who said it had no merit in value-for-money terms.

Mr Mills told the inquiry last week that he wanted to "kill off" the proposal and viewed it as part of a tactical plan to stall the cost controls.

Civil servants' attempts to curb the cost of the RHI scheme were met by "resistance" by some DUP figures, says Mr Stewart.

The DETI minister's adviser Mr Cairns suggested to Mr Stewart that the addition of cost controls should be delayed as their introduction in October could lead to a spike in applications to the scheme.

But Mr Stewart countered that there was already an increase in demand and that a well-informed industry would keep demand high.

He tells the panel his view at the time was that delay would not make matters better at best a "spike might turn into a plateau".

The witness says he thinks the proposal to delay the changes was based on a DUP view, not from the adviser himself or the minister - he believes Mr Cairns saw the need for cost controls but needed something to sell to the party.

He attributed the delay to the party viewing the RHI scheme as good for industry, he adds, and you "don't hobble a good scheme any more than is necessary".

Mr Stewart says he assumed that the submission regarding the changes to the RHI scheme did not actually reach the DETI minister until the end of August 2015.

Instead, he believed it was only ever in the hands of his Democratic Unionist Party (DUP) ministerial adviser up to that point.

He says he took that view because the adviser Timothy Cairns returned to civil servants with suggested amendment that "were at a level of detail I wouldn't have expected... from a minister".

And he adds that Mr Cairns didn't say that he'd discussed the submission with Mr Bell.

"I got the impression that the suggestions that were coming back were... as a result of party considerations."

Evidence the inquiry has seen shows that Mr Cairns had been discussing the changes with another DUP adviser Dr Andrew Crawford, who had put forward possible changes to it.

Inquiry barrister David Scoffield QC (below) picks up the questioning at the point where he finished yesterday, with a submisson that was sent to the enterprise minister Jonathan Bell on 8 July 2015 - it's a key document in the story of the RHI debacle.

In it, Mr Bell was informed about the emerging problems with the scheme and how DETI officials proposed to deal with them.

Image source, RHI Inquiry

An early version, which clearly stated the projected expenditure in 2015-16 was £23m - twice the available budget - was amended to remove the cost controls warning.

A paragraph which suggested that a funding shortfall might have to be paid from other DETI spending commitments was also completely removed.

Mr Stewart says the submission was "inaccurate in its description of funding and in particular its description of the risks" associated with that.

It's a second day in the witness chair for Chris Stewart, the deputy permanent secretary at the Department for the Economy.

The department was formerly the Department of Enterprise, Trade and Investment (DETI) and it set up and ran the RHI scheme.

Image source, RHI Inquiry

Mr Stewart joined DETI in August 2014 and his deep involvement with the RHI scheme appears to have started in May 2015, when he began helping to manage it as it started to run out of control.

You can find his two witness statements on the inquiry's website here, external and here, external.

His evidence yesterday was eye-catching at times and he'll be answering many more questions from the inquiry's senior counsel David Scoffield QC today.

BBC News NI

A top civil servant told the inquiry that he felt officials were treated as the "opposition" by DUP advisers as they tried to curb the rising cost of the flawed RHI scheme in 2015.

Chris Stewart said he felt the civil servants, advisers and their ministers were not "on the same team" as they struggled to rein in spending.

Mr Stewart, one of the top tier of management in Stormont's economy department, acknowledged that it was a big statement to make.

And he said had there been greater trust between officials, advisers and the minister they could have thrashed out an approach that might have been much more effective.

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 developing political crisis surrounding the scheme.

The RHI Inquiry began in November 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 will look 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 2015
• the scheme's closure

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 Democratic Unionist Party (DUP) leader Arlene Foster faced calls to resign from her role as Northern Ireland's first minister in December 2016.

Image source, Pacemaker

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, 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 it 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.

The most recent estimate for the overspend was set at £700m, if permanent cost controls aren't introduced.

The massive overspend bill 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... and 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.