Image source, PA

Mr Scoffield asks if there was any advice on "where you draw the line" on sharing information about the RHI scheme that "might have a significant financial effect" on those within the renewable energy industry.

Mr Mills says he can't think of anything specific but agrees that there is "scope for more guidance here".

"The line needs to be drawn much more tightly than was in that case," he adds.

Mr Scoffield puts it to Mr Mills that from mid-June 2015 there was already a sense in DETI that industry figures had become aware of quite specific proposals for changes to the RHI scheme.

"I had assumed the leak came from the political side... meaning the [DUP advisers]," responds the witness, adding that some of the evidence that has come before the inquiry points to that.

Image source, RHI Inquiry

Regarding his suspicions at the time, he says: "It would have come from a general sense of distrust I would have had of that political system's ability to keep things confidential."

"The very job of the [adviser] is influencing people and information has a currency," Mr Mills adds.

Even before the minister was informed of plans to change the subsidies on offer through the RHI scheme in October 2015, DETi staff had been briefing people in the renewable energy industry about the changes.

Mr Mills says they would accept they'd "gone too far with some of this information".

Mr Scoffield says DETI knew it had an urgent budget problem, it had been told by finance officials to stop spending money and was aware there was likely to be a spike when it moved to introduce tiering,

He puts it to Mr Mills that it was "more than naive but simply unwise" to give the industry advance notice.

"It is, in hindsight," says the witness, but he adds that officials could be put in the position of having to lie.

Mr Bell was "more enthusiastic about renewables than the previous regime" - that being Arlene Foster's leadership of DETI - according to Mr Mills.

The witness had a meeting with Mr Bell's adviser to inform him of difficult decision that would be coming up on the RHI scheme.

He explains that Mr Cairns' attitude was that if something needed to be done it would be but civil servants shouldn't make things any more difficult for the minister.

Image source, DUP

Mr Bell was "too happy to take instruction from" his DUP adviser, says Mr Mills.

In his witness statement, Mr Mills also says the minister "appeared to be less involved in decision making than... he should have been".

He also agrees that Mr Bell's adviser was keen to emphasise that the minister "should take direction from the party centrally", as Mr Scoffield puts it.

It was "stressed" by the enterprise minister's adviser that Mr Bell "was not someone who became involved in the detail of issues, undertook lengthy preparation or was good at absorbing complex submissions", writes Mr Mills in his witness statement.

The witness says he found that to be correct and he "wouldn't say on RHI that [Mr Bell] had deep knowledge".

Image source, Pacemaker

He compares Mr Bell with his predecessor Arlene Foster, who he says had a "very good knowledge of energy".

"Like everybody, Jonathan Bell would've struggle with the complexities in and around energy issues," adds Mr Mills but he acknowledges that the then DUP MLA had only be in the role for a few weeks at that point.

In early June 2015, a DETI delegation travelled to London for a meeting with the then energy minister Amber Rudd.

On the evening before the meeting, minister Jonathan Bell and his special adviser, Timothy Cairns, had what's bee described as a "heated discussion" at an Indian restaurant.

The disagreement continued at a pre-meeting breakfast attended by the two men plus Mr Mills and DETI permanent secretary Andrew McCormick.

Image source, Getty Images

In his witness statement, Mr Mills says Mr Bell was saying "words to the effect that: 'I am the minister, I can do anything'" and Mr Cairns said decisions had to be cleared by the party.

The argument "seemed to be on the principle of who called the shots," Mr Mills tells the panel.

On the way to the meeting, Mr Bell said Mr Cairns would not be attending and the adviser subsequently went on sick leave.

In his statement, Mr Mills says: "Initially I assumed the illness to be political in nature."

Image source, RHI Inquiry

Civil servants should share uncertainty about projects with ministers "early and often", says inquiry panellist Dame Una O'Brien.

As a former Whitehall permanent secretary, she's worked with government ministers and brings significant civil service experience to the inquiry.

Ministers "carry the can if uncertainty eventually plays out", she says, and therefore they have a need to know what is happening in their department.

Relevant emails from 2011 about the RHI scheme funding arrangements had been filed in the DETI's electronic archiving system but they were not uncovered when officials were trying to understand those arrangements in the spring of 2015.

Mr Mills explains the limitation of the system: "It's great if you know what to look for."

Image source, RHI Inquiry

Sir Patrick says the implication of that is that the archiving system isn't fully understood, isn't working properly or hasn't been properly explained to the people who are supposed to use it.

He says that if that's right it's "a terrible indictment, I'm afraid, of the department's administrative services".

"Well, I wouldn't say knowledge transfer was a strong suit," says Mr Mills.

Mr Mills agrees that changes made - at his direction - to the wording of the July 2015 submission to the minister made it "far less explicit" about the consequences but it didn't "alter the central message".

He says: "Now I'm being told I was trying to cover this up in some way."

Image source, RHI Inquiry

But Sir Patrick calms matters, saying: "Let's not look at covering it up or loaded words like that.

"It is a question of what [the minister] was being told - and he was not being told what he should've been."

Mr Mills accepts responsibility for the changes and adds: "Of course, at the time I no thought that this would ever have had the significance it would have."

The enterprise minister would've been relying on DETI civil servants to provide a submission with "accuracy and truthfulness" to inform his decisions on the RHI scheme, says Sir Patrick.

But the document he got was "inaccurate" and Mr Mills should have taken care in putting it together.

"Otherwise, how can [the minister] make an informed decision?" asks the chairman.

Image source, Getty Images

Mr Mills rejects Dr MacLean's earlier comments about "hiding" the figures, saying there was "absolutely no intention to conceal or try and make the picture rosier for the minister".

"I can't imagine what advantage there would've been to us to do so."

In an 8 July 2015 submission, DETI minister Jonathan Bell was told the funding for RHI scheme was coming from the Treasury and "does not impact directly on Northern Ireland's departmental budgets".

But the DETI officials knew there would be a 5% penalty imposed on the department's own budget if there was an overspend.

Changes were made to the original wording of the submission - they were closely examined this week when the inquiry heard that there had been a "significant downplaying of the need for concern" in what was sent to the minister.

Image source, RHI Inquiry

Mr Scoffield asks Mr Mills whether what the minister was being told was accurate.

"In hindsight it absolutely should have had something like 'except there might be a 5% hit' in there," the witness replies.

He says that in July DETI did get all of the money, with no penalty.

Inquiry panel member Dr Keith MacLean says that when you look at the figures shown elsewhere in the submission: "You were effectively hiding the fact that you already knew... that at the very least the impact on your [department's budget] was going to be £600,000."

Mr Mills' understanding of the budget arrangement for the RHI scheme is the subject of close scrutiny.

The Treasury supplied the money for the initiative to DETI on the basis that the Stormont department would would need to cover any overspend it might make in future years and there would be a 5% penalty imposed.

Image source, RHI Inquiry

Inquiry chair Sir Patrick Coghlin presses Mr Mills, saying: "I only ask you this to see how much of this you had grasped."

The witness says at the time he "just focused" on the penalty and he doesn't think he "was of the opinion" that any money that DETI had spent beyond the scheme's budget would also have to be repaid.

Mr Scoffield says that notion of DETI only having to "pay a very small proportion" of any overspend "as a slap on the wrists" would've been "remarkably generous on the part of Treasury".

Mr Scoffield picks up the threads in spring 2015, when DETI's energy branch was trying to get a grasp of how the RHI scheme was being funded.

The budget for the scheme came in the form of annually-managed expenditure (AME) - it's usually the funding method used for programmes that are demand-led - welfare, tax credits or public sector pensions, for example.

Crucially, any overspend on AME funding is usually picked up by the Treasury.

Image source, Getty Images

The RHI scheme's funding has been described as "AME with strings" - in that overspends would have to come out of the Northern Ireland block grant.

The unusual nature of the RHI funding was set out in an email from a Treasury official, sent to DETI in 2011.

This email had disappeared from the DETI radar and was only rediscovered in March 2015 when officials were desperately trying to obtain clarity on the scheme's budget.

Back for an all-day session before the inquiry, John Mills takes his place in the witness chair.

A reminder of his involvement - from January 2014 he was the boss of DETIs energy division, which had set-up and was running the RHI scheme.

He held that role during a key period in the lifetime of the initiative, including when things spiralled out of control and the vast overspend occurred, and he's since left the department.

Image source, RHI Inquiry

Mr Mills has submitted a few written witness statements to the inquiry - you'll find them here, external and here, external.

He's back this morning after giving evidence yesterday afternoon when he was questioned about his knowledge about the scheme's budget.

The inquiry's senior counsel David Scoffield QC is posing the questions.

BBC News NI

An attempt to stop a spike in applications to the RHI scheme led to a flood of people signing up to the lucrative initiative, the inquiry was told.

The inquiry panel heard that Stormont's enterprise department got involved in informal sharing of information with the energy industry in the summer of 2015 about plans to cut the subsidies on offer.

Image source, Pacemaker

The industry passed that information on widely to potential applicants, leading to almost a thousand biomass boilers being accredited on the scheme in the three months before the cuts took effect.

John Mills, the head of DETI's energy division, said there had been "naivety" about the extent of the information shared by his officials.

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.