Mr Scoffield admits defeat - he hasn't been able to finish his list of questions, so Mr Sterling will have to come back next Tuesday.

Inquiry chair Sir Patrick Coghlin and the inquiry's legal team are mindful that the witness may have other things to attend to - such as running the country in the absence of a Stormont.

Image source, AFP

In the meantime, a quick look out the window reveals that there's been a quare stretch in the evenings since we were last up here at Stormont, so we're off to make the most of it.

Join us at 09:45 tomorrow the morning for more live coverage from the RHI Inquiry.

BBC News Northern Ireland

Northern Ireland's most senior civil servant said he wished he had been "more inquisitive" about the RHI scheme while he was overseeing the department that was running it.

David Sterling, now the head of the civil service, was the permanent secretary at DETI from the outset of the scheme until 2014.

Image source, Press Eye

He said he only got involved in projects day-to-day if three potential trigger points were reached, but that didn't happen in the case of the initiative while he was at the department.

He also said "more credence" should've been given to warnings that were raised about the flaws in the scheme and the dangers it posed to the public purse.

Mr Sterling says he is not satisfied with the advice that DETI civil servants gave to the minister Mrs Foster regarding the two options for the RHI scheme in June 2011.

"In this particular case, the advice could have been clearer... should have been clearer," he adds.

Image source, DETI

Asked if he bears any responsibility for that, he accepts that he has "ultimate responsibility for what goes on in the department".

As a permanent secretary, he has to "delegate a certain responsibility to lower levels" in the department and make sure that "people have the necessary competency and resources to deliver".

But he adds he can't be expected to "check that everything on every occasion is absolutely right - that would not be an efficient way to run an organisation"

The witness says his role as adviser would be to ensure that the minister "was in full possession of all the relevant facts".

But Sir Patrick playfully goads Mr Sterling: "With hindsight you wish we had adopted the capital-based [challenge fund] fund."

Image source, RHI Inquiry

Mr Sterling admits that that's what he told the Assembly's Public Accounts Committee.

"It's not fair to tax you with it completely but you did say it," the inquiry chair grins.

"I did say it, and to be honest I did mean it, but it was more reflecting on the difficulties we had with this scheme," says Mr Sterling.

Image source, Getty Images

Civil servants at Stormont "seek to maximise the amount of resource you can get from the Treasury", says Mr Sterling.

"That's the way in which we operate," he goes on, adding that departments are "looking always to see is there more [money] we can draw down".

There was a "strong preference" at DETI for the RHI scheme to be an incentive-based one, says Mr Sterling, rather than an up-front grants option that represented better value for money.

Asked where that preference came from, Mr Sterling says he "won't put words in Mrs Foster's mouth" but there was a "collective view" between her, her adviser and her officials that an incentive scheme would be the "best solution".

Mr Sterling says it was based on the fact that it the same model that was used in the GB scheme and "something that's good enough in England should be good enough here".

It was viewed that it would offer guaranteed funding over time that would ensure the sustainability of the renewable energy sector in Northern Ireland.

But Mr Sterling adds that there should've been "greater clarity and less ambiguity" for Mrs Foster in deciding which model to choose.

Mr Sterling had the ultimate responsibility for any advice given by DETI to the minister.

Mr Scoffield takes him back to June 2011 and a submission to Mrs Foster, in which she was asked to decide what type of model she wanted for the RHI scheme - either a grants-based model, known as a challenge fund, or an ongoing subsidy model, which was ultimately adopted.

In a report at the time by external consultants at CEPA who assessed both options, the challenge fund model was projected to produce the highest amount of renewable heat at the least cost.

Image source, Getty Images

There was also a overall cost difference of £218m between the two options, in favour of the challenge fund.

Asked if he aware of the disparity in cost at the time the submission was going to the minister for approval, Mr Sterling says he was not, and he only recently became aware of the disparity.

He confirms that he would have not read the CEPA report at the time.

There are "many things" about the RHI scheme that Mr Sterling "can't satisfactorily explain" and "appear to be counter to what I would normally have expected from the people involved", he says.

Referring to the points that Mr Sterling has made today about how he "took comfort" from the fact that DETI was following in the footsteps of DECC in introducing the RHI scheme, Dr MacLean says that assurance was "misplaced".

He notes that in "a whole series of areas DETI had decided not to follow" the model of the GB scheme.

Image source, RHI Inquiry

Those include not introducing cost controls to the initiative early on, and then appearing to prioritise the opening of the non-domestic scheme over adding cost controls to the domestic initiative in 2015.

Both of those decisions were taken against the advice of other key players.

Dr MacLean says that none of the decisions were documented, nor were they referred to Mr Sterling, and he adds: "It's more than an isolated incident that we're observing."

Image source, Getty Images

Inquiry panellist Dr MacLean makes the point that DECC - which had 77 people working on the RHI scheme - had decided to defer the opening of the domestic initiative in favour of getting cost controls in place in the non-domestic.

He says it's interesting that DECC's 77 staff "was described as a limited resource", especially when compared with the one full-time and one part-time staff member working on DETI's initiative.

In his written statement to the inquiry, John Mills - who succeeded Ms Hepper as the head of DETI's energy division in January 2015 - has indicated that there was a ministerial imperative to get the domestic RHI scheme opened that year, meaning the review of and important changes to the non-domestic initiative were set to one side.

Asked if he was aware of that, Mr Sterling says he can't recall any discussion in which there was a "trade-off" whereby the domestic scheme was prioritised ahead of the non-domestic.

Sir Patrick Coghlin says there was a number of key matters that could have indicated how well the RHI scheme was controlled. Those were:

• a review of the scheme
• a monitoring board to assess the scheme on an ongoing basis
• a re-approval process for the scheme

He says that Mr Sterling placed an enormous responsibility and trust in DETI's casework committee, which was given an undertaking that the scheme would be reviewed and that there would be a monitoring board.

Neither of those things happened.

Image source, Getty Images

"How come, in view of the robust control measures, all of those things disappeared?" he asks.

"That is the conundrum here," says Mr Sterling, adding: "I can't understand why the significance and importance of the review seems to have been lost as time passed during 2014."

Sir Patrick is unimpressed: "It was lost as far as you were concerned - you didn't even know it was due. That's what you told the [Public Accounts] committee."

There should be a "culture" within the civil service that people can feel free to "flag up problems when they arise" so that managers "can deal with them promptly", says Mr Sterling.

Image source, Getty Images

He tells the inquiry that he would generally tell his teams: "'Don't try and hide it or hope it'll go away, because it becomes much more difficult to resolve.'"

Risk was taken "seriously within the department", he adds, but he says that he can't explain why the risks associated with the RHI scheme "weren't managed and monitored more frequently".

We're up and running again after lunch.

Mr Sterling was copied into a large number of submissions to the minister concerning or relating to the RHI scheme, and he explains that how he dealt with them depended on the nature of them.

In the case of one in March 2012 in which the RHI scheme team was seeking to proceed with the project's business case, he says he would've read the summary, the recommendations and who produced the paper.

Image source, Getty Images

"I know in that case I didn't read all the annexes - I would have taken comfort from the work of the casework committee," he says.

Mr Sterling confirms that it would have been impossible to read all of the hundreds of emails he received on a daily and weekly basis.

In some cases he would merely have looked at the title of the email and decided whether or not he needed to look at it.

It is "concerning" that ministers and civil servants have "less of a willingness or desire to have things recorded" in order to frustrate freedom of information requests, says the inquiry barrister Mr Scoffield.

He asks to what extent a conscious decision has been taken to "dumb down or reduce the minuting" or whether or not that is appropriate.

Image source, RHI Inquiry

Mr Sterling says he would've "encouraged openness" and "fluid engagement" between ministers, their advisers and their civil servants.

A consequence of that, he says, is that it becomes "much more difficult to apply the rigid disciplines of minuting every meeting".

If the Stormont executive ever returns, he says he'll have to have a "very clear discussion with ministers", telling them that "we do need to get back to a period where we have good, basic administrative practice in place".

Mr Sterling says he has spoken to a number of permanent secretaries across Stormont departments and they have agreed that the good practice of minuting meetings with minsters has "largely lapsed" under devolution.

He says "the pace of everyday life has increased exponentially".

Image source, Getty Images

It became common for him just to ask for "a quick word with the minister".

However, he says that it is important that "a key decision on policy would always, or should always, still have been reflected in a (written) submission".

Mr Sterling explains that meetings with ministers sometimes go unminuted because "ministers like to have safe space where they can consider difficult things, think the unthinkable and not necessarily have it all recorded".

He says that is a particular feature of how Northern Ireland's devolved administration has worked.

Image source, Getty Images

The two largest parties - the Democratic Unionist Party (DUP) and Sinn Féin - have been sensitive to criticism, he explains.

"We got into the habit of not recording all meetings on the basis that it is safer sometimes not to have a record that, for example, might be released under freedom of information," he adds.

Inquiry chair Sir Patrick Coghlin raises the point that a decision was taken within DETI to plough ahead with opening the RHI scheme without cost controls even though that presented a "real danger".

That decision was taken during a conversation between the minister and DETI's energy boss Ms Hepper and was not recorded anywhere, so Sir Patrick wants to know if that was "an appropriate safeguarding of a decision".

Image source, RHI Inquiry

"A very good question," replies Mr Sterling.

He goes on to say that he wouldn't have considered Ms Hepper as an "excessive risk-taker" and her decision would've been taken on "a balanced assessment of the risk".

But he adds that it's a fundamental understanding that there should've been a record taken of them meeting between Ms Hepper and Mrs Foster, detailing what decision had been reached and why.

Having heard from Mr Sterling a detailed outline of the controls for the RHI scheme within DETI that he would have "taken comfort from", Mr Scoffield says: "Having listened to the list, I think the simple question is: 'How did it all go wrong?'"

The witness says he has "agonised about this this for two years now, and my conclusion is that there wasn't one thing that went wrong - there was a variety of things that went wrong".

Image source, RHI Inquiry

"I suspect once the scheme was up and running there was a sense that it had developed a bit of momentum," he says.

The witness says DETI officials should have "drawn a link" between the warning signals, should then have "paused and said: 'Hold on, I'm not sure that this is right'".

An internal scrutiny panel - known as a casework committee - assessed and approved the RHI scheme in March 2012, and Mr Sterling says the process was something he would've taken comfort and confidence.

The three men who sat on the panel are people he has the "absolute highest regard for" and took their responsibility "very seriously and would've applied a considerable amount of diligence" to their work.

Image source, Getty Images

Each of them have given evidence to the inquiry, and Mr Sterling says it's "quite clear" from what they've said that they felt it was "a bit beyond what they would normally be comfortable dealing with".

"Nobody flagged up that general concern to me," he says.

He says he would've also "taken comfort" from the Department of Finance and Personnel having give its approval for the scheme.

Mr Sterling says he relied on "people and systems" to make sure the RHI scheme was run properly.

But he accepts that there were problems from the outset, when the external consultants who were hired by DETI to draw up the foundation for the scheme "didn't get it completely right".

Image source, RHI Inquiry

With the benefit of hindsight, he says DETI needed some "additional competency so we could adequately challenge the advice we were getting from the consultants".

He refers to the case of DETI economist Sam Connolly (above), who assessed the scheme and gave his approval of it in terms of value for money, but who told the inquiry in January that the "concepts and sums of money" involved in it were "beyond me".

Mr Sterling says he wants to understand why Mr Connolly "wasn't comfortable with raising those concerns" that he wasn't capable of handling the scheme and why he didn't ask for help.