It's put to Mr Sterling that there were a number of "flags" that the RHI scheme was not going to reach its targets, particularly in 2013.

Mr Scoffield mentions a report by the consultants at CEPA, which warned that the scheme was unlikely to help meet Northern Ireland's target of producing 10% of the region's heat through renewable sources by 2020.

Mr Sterling says that should it have been flagged up to DETI's senior management.

"Until I started looking this all again I wasn't conscious that there were flags in 2013 which were suggesting that we wouldn't meet the 2015 and 2020 targets," he adds.

Dr MacLean says that given that those failures could have resulted in substantial fines from the EU, shouldn't his system have been flagging this up to him?

"Absolutely," the witness replies, adding that ministers would be "unforgiving" if Northern Ireland had been fined for missing the target.

Mr Scoffield suggests that the evidence Mr Sterling has given so far suggests that DETI did "more things than you should, not as well as you should".

The witness says the RHI scheme was the only one he's aware of in which there were "major failings".

He tells the inquiry that civil servants are "proud of what they do" and will "first and foremost, try to find a way of doing" what they are asked to.

He adds that what has happened in the RHI scheme "should cause us to sit back and say: 'On occasions, should we be a little less can-do?'"

Stormont ministers were "criticised on any occasion" for now drawing down money that was made available to them from Westminster or Europe, says Mr Sterling.

Mr Scoffield takes his evidence to suggest that it is "difficult to say no to a minister" who wants a project to be put in place.

Image source, AFP

The witness says that he would've advised staff that "you don't say 'no' to a minister".

He says an alternative method would be to advise that a project would be difficult or say that it could be done but might take a longer time to set in motion than the minister had hoped.

But he adds: "We would normally have tried to find a way of ensuring the minister got the outcome she was looking for."

Mr Sterling says he told the Assembly's Public Accounts Committee that "we would have been better off if we hadn't done it".

But he says that people need to look at the context at the time when there was a budget of £25m for the scheme - "here was an opportunity to inject a significant some of money into the economy" - particularly if it was seen that the money could be lost if it wasn't spent.

Image source, RHI Inquiry

Panel member Dr Keith MacLean (above) says it doesn't seem there was any thought given by DETI to not going ahead with the scheme.

"Once we'd started there was a momentum built and I think there was a desire: 'Yes, let's do this'," Mr Sterling says.

Inquiry chair Sir Patrick Coghlin says "something went very badly amiss" at DETI in allowing the setting up of the RHI scheme to be effectively left to one civil servant who did "all of the donkey work" but didn't have any qualifications in energy or economics.

Mr Sterling says he accepts that and recognises that the department looks "naive" as a result of that.

Image source, Getty Images

But he adds that staff who are "competent in many ways but not specialist in their area" is "characteristic in many teams across the Northern Ireland Civil Service".

Staff need to be encouraged, he says, to speak up if they are being asked to "do something that is beyond their capability".

DETI "didn't full appreciate all the risks attached" to running the RHI scheme, accepts Mr Sterling.

He reiterates his earlier point that he was "taking comfort" from the fact that a similar incentive initiative was running in Great Britain.

Image source, Getty Images

But, as we mentioned earlier, DETI made crucial changes to the model for the GB initiative, including deciding not to replicate cost control mechanisms - that was a move that proved to turn into a fatal flaw.

He says he "wasn't aware at the time that we were stripping out cost controls", and says he accepts that that was "a little naive".

In his role as DETI's permanent secretary, Mr Sterling was the principal adviser to the Arlene Foster, the department's minister.

Mr Scoffield asks if he can remember ever personally giving advice on the RHi scheme to the minister.

"No, nor do I recall being asked for any particular advice on the scheme," he replies.

Image source, Reuters

He outlines the process of giving advice to the minister and says that they sometimes joked with Ms Hepper DETI's energy boss, that "she produced the longest submissions in the department".

"The quality of submissions from energy division would probably have been regarded as probably the best in the department," he adds.

Asked if there were any occasions when the RHI scheme should've been drawn to his attention, Mr Sterling discusses three key points. They are:

• the concerns raised by the so-called whistleblower Janette O'Hagan after the scheme opened
• the concerns raised by lawyers involvedin drawing up the scheme's regulatons
• the concerns raised by the scheme administrator Ofgem

He says: "Piecing those things together there should been recognition that: 'Maybe there's something not quite right here - maybe we should pause.'"

he says that if the team had believed those concerns were valid, they should've been drawn to his attention or the attention of his David Thomson, who was one level beneath him in DETI's management team.

Mr Scoffield puts it to the witness that the RHI scheme was one where he should have taken "a more active interest".

There is a long pause before Mr Sterling says that he had taken comfort from eight or nine factors.

But he adds: "Knowing what we know now I probably should have asked more questions about some aspects of that."

Image source, RHI Inquiry

He says that in his defence it was "a busy department - a lot going on".

"I'm prepared to accept now that I should perhaps have been more inquisitive and got more actively involved," he adds.

When pressed by Mr Scoffield, he says "I wish I had the foresight at the time" to have recognised the lack of resources and proper management programme for the project.

At the time of the setting up of the RHI scheme, the economy was the "top priority" for the Northern Ireland Executive after about 10.5% of the value of the region's economy had been lost, says Mr Sterling.

DETI was being asked to "punch well above our weight", he adds, given that it was a small department with a small amount of money to work with, in the context of the overall Northern Ireland budget.

Image source, RHI Inquiry

Mr Sterling says his "particular focus" was on jobs creation in Northern Ireland and to "get money flowing from the banks" to allow businesses to invest.

He says that "knowing what I know now, we should've invested more" time and resources in the part of DETI that was dealing with the RHI scheme.

"It's a matter of profound regret on my part that I didn't recognise that," he adds.

Mr Sterling says he can "certainly understand why the public" might think it's strange that he didn't get more deeply involved in the RHI scheme from its early days.

But he adds that "the way we do business" is that the department makes sure the "right people and the right process in place" for such projects to work effectively.

Inquiry chair Sir Patrick Coghlin intervenes to observe that the RHI scheme was the first of its type in Europe, and probably the world, following on from the similar initiative in Great Britain.

Image source, RHI Inquiry

"It was volatile; it was unpredictable; it was subject to changing assumptions," and was "funded in a highly unusual way", Sir Patrick says.

He notes Mr Sterling's earlier comment that: "It was not so novel that I should take a personal interest."

"That's correct," says Mr Sterling, added that that was his view at the time.

Mr Sterling didn't get involved in the RHI scheme when it was being conceived because none of "three triggers" applied. Those were:

• if he been asked by the staff working on the project to get involved
• if the minister had asked him to keep an eye on it
• if he saw an issue he had a concern about

Image source, Getty Images

He accepts that it was a "novel scheme" but says that he would've "taken considerable comfort" from the fact that the scheme was broadly based on a similar initiative being rolled out in Great Britain by the The Department of Energy and Climate Change (DECC).

But, as we now know, DETI made crucial changes to the scheme that took it in a different direction to the one operated by DECC.

The RHI scheme "wasn't one that I felt any any early stage I should get personally involved in", says Mr Sterling.

That's because he felt that DETI's energy division had a "track record of managing a busy caseload".

Image source, Getty Images

Even though it was a division that was "stretched and had resource pressures", it was made up of people who had "considerable competence", he adds.

He had formed a "very favourable opinion" of the boss of the energy division Fiona Hepper's ability to "manage a busy workload".

Ms Hepper has appeared before the inquiry a few times so far - she was one of a small number of civil servants most deeply involved in the set-up of the scheme.

Mr Sterling is asked about the extent of his involvement in the key developments of the scheme.

He gave a number of approvals to appoint business consultancy Cambridge Economic Policy Associates (CEPA) for work, and Ofgem as the scheme's administrator.

Image source, RHI Inquiry

He also became involved when there was a "difference of view" between the head of DETI's energy division Fiona Hepper and the department's finance director Trevor Cooper in late 2013.

Mr Sterling was also copied into submissions to DETI minister Arlene Foster, and received a monthly update as part of a headline points report from energy division.

Mr Sterling says he "would hold to" the "broad thrust" of what he told Stormont's Public Accounts Committee in November 2016.

We covered that appearance at the time and if you fancy a glance over what Mr Sterling said you can read it here.

Image source, Getty Images

But he says the one thing he's "more conscious of now" is that he thought that it "would've been easier to change the scheme following a review than I now understand".

"That's probably the most significant thing that I've learnt that I didn't know then.

"At the time I wasn't clear just what the full extent of the process that would be needed before change could be put in place, including, for example, the consultation, etc."

Mr Sterling makes an affirmation to tell the truth in his appearance before the inquiry.

Image source, RHI Inquiry

The inquiry's senior counsel David Scoffield explains that Mr Sterling was permanent secretary at DETI from October 2009 to June 2014.

You can find Mr Sterling's written witness statement to the inquiry here, external and here, external.

The head of the Northern Ireland Civil Service since his appointment last summer, David Sterling has a lot on his plate, especially in the absence of ministers at Stormont.

He joined the civil service in 1978, rising up the ranks and eventually heading some Stormont departments, including the Department of Finance.

At the time of the set-up of the RHI scheme, he was the permanent secretary - the top civil servant - at the Department of Enterprise, Trade and Investment (DETI).

Image source, Press Eye

In November 2016, he appeared before the Northern Ireland Assembly inquiry to answer questions about the cash-for-ash debacle and denied trying to "duck responsibility" for his role in it.

"I'm not seeking to pass the buck," he told the Public Accounts Committee. "I accept responsibility for failures which occurred during my time."

But he told the committee that he had "no satisfactory answer" for why a planned review of the initiative in January 2014 did not happen.

BBC News NI's business correspondent Julian O'Neil profiled Mr Sterling last June, and you can read that here.

BBC News Northern Ireland

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, Press Eye

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, more than a year 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.