Image source, AFP

Sir Patrick Coghlin tells Mr Mills that the inquiry knows it's "not easy" to spend as long as he has in the witness chair.

But it has a "duty to the public" to ask questions, he says, and he thanks the witness for answering them.

Mr Mills is unlikely to have to return to Stormont to give more evidence.

Tomorrow's witness is Tom Forgrave, a poultry farmer and chair of the Ulster Farmers' Union's poultry committee - join us from 09:45!

BBC News NI

A key meeting that led to a disastrous delay in RHI cost controls was driven by an unelected DUP adviser rather than his minister, the inquiry was told.

The decision to push back the introduction of subsidy cuts by a month was taken at a 15 minute meeting in August 2015.

Image source, Pacemaker

The extra four weeks allowed an extra 800 boilers to be accredited to the scheme, adding a massive sum to the overall cost.

The inquiry heard the idea for the delay was at the behest of DUP adviser Timothy Cairns even though his minister Jonathan Bell (above) was in the room.

Standard procedure for closing the RHI scheme would have included public consultation but this was dispensed.

Mr Mills says he believes the decision was taken once it came to the attention of Sinn Féin.

Image source, Sinn Féin

He says some of the earlier submissions were labelled 'First Minister' whilst later versions were marked 'Office of the the First and deputy First Minister', implying they had been seen by the DUP and Sinn Féin.

The witness says he thinks he was first alerted to the involvement of Sinn Féin advisers by DETI's top civil servants Dr Andrew McCormick and Chris Stewart, "who maybe got it from Timothy Cairns".

"Things had moved up a notch," he adds.

There was an understanding at DETI that the Democratic Unionist Party had "rescinded" Jonathan Bell's approval for the RHI scheme to close, says Mr Mills.

He claims that DETI officials were told that the decision "was in the hands of party officers".

He says he "probably thought the worst at the time" and "very disturbing reasons for it were probably in my head".

Asked what those were, he says he believed it was "to allow people to make applications before closure".

"There was no reason for the decision [to close the scheme] to have not been taken."

"Information very quickly leaked out" about the planned closure of the RHI scheme in early 2016, says Mr Mills, and DETI was receiving calls from people looking for confirmation.

It even received a call informing it that Moy Park had instructed its farmers to act quickly and get their applications in.

In his witness statement, Mr Mills says that "stakeholders appeared to have greater knowledge than officials" about what was happening.

Explaining that, he tells the inquiry that it was his assumption that information was being provided to the industry by ministerial advisers but he acknowledges that he doesn't have evidence to back the claim.

But he adds: "I couldn't see how else the information was getting out... it simply couldn't have been officials."

Jonathan Bell was told in a ministerial submission that DETI had "no choice but to close" the RHI scheme "as soon as possible.

"This may seem drastic," it stated, "but the funding risk is very serious indeed."

In response to the submission, Mr Bell's adviser Tim Cairns asked if the department could "consult or inform" the Ulster Farmers' Union and others within the energy industry that the scheme was to close.

Mr Mills responded that it "would not be appropriate to consult" with the industry given that the scheme had run so far out of control.

Mr Mills says he was concerns that the adviser had made that request.

Matters move to the closure of the RHI scheme in 2016 and Mr Mills is asked about his involvement.

He makes the point that much of the decision-making was done above his level but he was instructed just before Christmas in 2015 that the scheme was have to be shut down.

Image source, Getty Images

"Chris Stewart (DETI's deputy permanent secretary)... sent me a note saying the game's up - we're going to have to pay for this," he says.

But over a month earlier, DETI civil servant Stuart Wightman had told Mr Stewart that there was no option but to close the scheme.

Asked why it took so long from that point for the instruction of closure to be given, Mr Mills says officials were making a "last-ditch attempt" to convince the Treasury to cover the overspend.

Two senior DETI civil servants had a role in drawing up the business case for the changes to the RHI scheme, even though they would later be sitting on an internal panel that would be scrutinising it.

Finance official Trevor Cooper and economist Shane Murphy approved the business case when it came before them in the role on the department's casework committee.

Image source, Getty Images

Mr Mills says that "wouldn't be in accordance" with how the process was supposed to work.

The department was "fully behind" the plan for the changes, which is "not consistent with the independent challenge" function of the committee

Sir Patrick says there was "no element of challenge, nor was there intended to be any element of challenge".

Yesterday, during the session with Stuart Wightman it emerged that DETI's claims for job creation linked to the RHI scheme would have seen four full-time jobs for every 12 biomass boilers installed over the 20 years of the initiative.

"Surely nobody believed that was happening?" Dr MacLean asks.

"The job creation from renewables always struck me as high anyway," Mr Mills says.

Image source, PA

Dr MacLean wants to know if the constant emphasis on value for money and the claims that RHI was "a money machine" had distracted the DETI officials from the need to stop the scheme.

"That's a valid point," says Mr Mills.

There was another, final delay in changing the subsidies on offer through the RHI scheme - it was pushed back from 4 November 2015 to 18 November.

That allowed many more applications to flood in, with resulting additional costs to the scheme, and Mr Mills put that down to financial and legal delays.

DETI's deputy permanent secretary Chris Stewart said in his statement to the inquiry that "even a two-week delay had very significant implications for the cost of the scheme".

Mr Stewart said the fundamental reason for the delay was the time taken to draft the legislation and it would have been better to concentrate on the subsidies rather than adding other measures as well - Mr Mills agrees.

Mr Bell's adviser was the one running the meeting at which the decision to delay the changes to the RHI scheme was made, agrees Mr Mills.

Sir Patrick Coghlin says Tim Cairns - in spite of his importance as a DUP adviser - was "not the democratically elected minister".

Image source, Getty Images

The inquiry chair asks if Mr Bell was simply "sitting there nodding".

"I don't dissent from your explanation, chair," says the witness, who tells him that it was Mr Cairns "doing most of the speaking".

There is disagreement between Mr Mills and the minister's adviser about who proposed a one-month delay to the changes in the RHI scheme.

Mr Cairns argues that it was Mr Mills who made the suggestion - Mr Mills argues the opposite way.

But in an email to Mr Cairns after the August 2015 meeting, Mr Mills wrote that the changes would come into effect from "the start of November, as you suggested."

Mr Mills says that wasn't challenged by the DUP adviser at the time.

What Mr Mills does acknowledge is that he didn't resist the suggestion of the delay, saying that he was "relieved that there was a chance" to get the changes approved: "I would've thought it was a concession worth making."

It was decided at a meeting on 24 August 2015 to approve the changes to the RHI scheme but a decision was also taken then to delay the changes by from October until November.

Mr Bell and his adviser attended the meeting with Mr Mills and the department's permanent secretary Dr Andrew McCormick.

Image source, Getty Images

But Mr Scoffield says the inquiry faces a problem - those who were there have "no clear recollection" of the meeting and, as has been a theme of the RHI debacle, no minutes or written record was made of what happened.

That goes against civil service guidance and Mr Mills accepts that there should've been a note taken that detailed what was discussed.

Dame Una O'Brien says ministers' private offices are busy places with officials constantly coming and going to check on the progress of their submissions and whether they've been seen by the minister.

She asks Mr Mills whether he would've considered going to the minister's private secretary to say he was concerned about the adviser's handling of the submission and to make sure the minster received it.

The witness says such an action would be seen as "going over the [adviser's] head" and he points out that there is a space on the submission folder for the adviser's signature.

Image source, RHI Inquiry

"The private secretary is not seen as someone who can unlock that process and make sure that the minister, who's the elected person, is made aware of the urgency?" asks Dame Una.

Mr Mills says that sounds more like his experience of governing in Northern Ireland during periods of direct rule from Westminster, before the devolution of power to the Stormont Assembly.

DUP adviser Tim Cairns was trying to delay or avoid a decision being made on the changes to the RHI scheme, claims Mr Mills.

By 20 August 2015, the changes to the RHI scheme had yet to be cleared by the minister, in spite of various reminders to his adviser about the urgent need for approval.

"It could hardly have escaped his attention that were were concerned," says Mr Mills.

Image source, Getty Images

In an email to the top civil servants in the department, Mr Mills said civil servants had "effectively done everything we can".

He also warned that even if the changes were rubber-stamped by the minister that week it would only give the industry six weeks of notice until they came into effect, opening up the risk of legal challenge.

Mr Mills set out to "kill off" Mr Cairns' proposal for an increased threshold of 3,000 hours of a heating system's annual use after which RHI scheme payments would be reduced.

The suggestion had been put forward to Mr Cairns by Dr Crawford.

He emailed Chris Stewart, DETI's deputy permanent secretary, to voice his objections.

Image source, Getty Images

Mr Mills made the point that the original proposal of 1,314 hours was taken from the similar RHI scheme that was running in Great Britain, and that it represented 15% of the maximum possible running hours for a boiler.

He pointed out that the poultry growing cycle meant that sheds were left empty for a third of the time awaiting the next batch of chickens.

DUP advisers adopted "stalling tactics" to slow DETI's planned changes to the RHI scheme in the summer of 2015, claims Mr Mills.

Suggestions to change some of the proposals were put forward, as the inquiry heard on Tuesday, and Mr Mills says that "my reaction to it was fairly cynical".

He also says it was "commonly thought" that Dr Andrew Crawford - who was Arlene Foster's adviser when she was the enterprise minister and follower her when she moved to Stormont's finance department in May 2015 - "was the person pulling the strings".

The two most senior civil servants at DETI felt that Dr Crawford's "influence was very great still in terms of decisions in DETI", he says.

And he adds that the then minister Jonathan Bell's adviser "more or less had to go with" Dr Crawford's suggestions.

Image source, Getty Images

At the end of July 2015, Mr Mills and DETI's deputy permanent secretary Chris Stewart met the minister's adviser Tim Cairns to discuss the planned changes to the RHI scheme.

He remembers that the discussion was "along the lines of: 'We need to get this moving'" but he felt there was a "deliberate delaying" of that on Mr Cairns' part.

He says Mr Cairns asked for a note outlining the changes that would be passed to the "party apparatus" including other DUP advisers.

With that, the inquiry breaks for lunch - back at 14:05.

Image source, Getty

Mr Mills says he wasn't aware that his officials were sharing such detailed information about the RHI scheme change to those in the energy industry in June 2015.

He became "much more aware" in August that year when the department's proposals for the scheme "became common knowledge".

"Given the level of knowledge [the industry had] it was very, very difficult to deny what was going on with the officials," he adds.

Civil servants' sharing internal information about the RHI scheme to major firms could lead to "valid criticism" from small businesses that they were put at a disadvantage by not receiving the same details, suggsts Sir Patrick.

He acknowledges that poultry producer Moy Park - which received advance notice of changes to the RHI scheme - is the "golden people of the agriculture area".

Image source, Getty Images

But he says DETI has a "responsibility" to make sure there's an even playing field for other firms.

Mr Mills agrees that there were some "handpicked players".

But he points out that Moy Park, for example, is one of Northern Ireland's biggest employers it would be "naive" not to engage with them on policy matters related to its work.