Chair Robin Swann sends Mr Sterling on his way and draws the PAC into closed session for a discussion with Northern Ireland Audit Office officials on the evidence they have heard today.

That's all from us for now, but we'll be back with Stormont Live tomorrow to cover the agriculture and justice committees.

We're off for a carol singalong up in the Great Hall here at Parliament Buildings, where the Christmas tree lights are all set for the big switch-on shortly.

Goodnight for now!

SDLP MLA Daniel McCrossan (below) asks what parts of the public sector will be hit as a consequence of the financial disaster the scheme has created for the executive.

Mr Sterling does not answer that specifically, but says a £405m hole will have to be plugged over the 20-year lifetime of the scheme.

He adds that the "£20m a year pressure" is a "real and present challenge".

"Things are being worked on" between the finance and economy departments to minimise the cost of the scheme, he says, adding: "I really can't say any more than that."

DETI's energy division, which drew up the RHI scheme, had "a lot of very difficult challenges, a lot of very complex issues", Mr Sterling says.

Image source, DETI

But staffing levels were increased from 29 to 38, he explains, in response to a question from the DUP's Gordon Dunne.

In spite of that, Mr Sterling says staff were "still stretched" by the workload.

Mr Kearney asks if the fundamental design flaws of the RHI scheme have left it open to "aggressive commercial exploitation".

Mr Sterling accepts that failings "left the scheme open to being gamed".

David Sterling's performance before the PAC today has been "very disappointing", according to Sinn Féin's Declan Kearney.

He says the permanent secretary seems "to have worked with two parameters".

"One: you don't know; and second of all: there's a fact-finding exercise in play".

"Therefore, you're fettered in your ability to analyse this situation and give us a view in what has gone wrong," Mr Kearney says, appealing "in the vain hope" for more guidance from the civil servant.

In response, Mr Sterling insists he has to be careful that he does not say anything that could suggest he has "reached a judgement" on the conduct of those who were involved in the RHI scheme.

DUP MLA Trevor Clarke says Mr Sterling's insistence that he cannot remember aspects of his role in relation to the RHI scheme "makes it difficult" for the PAC.

"There's an awful lot going on within the department that you're not aware of," he adds.

Mr Sterling says he has "huge sympathy" with the committee, adding: "You probably do think: 'How did he not know what was going on?'"

"I would love to be on top of every detail of every scheme, every project, but I can't be," Mr Sterling says.

"It is difficult to explain here in isolation just how this could have happened."

The handover process during a change of permanent secretary in executive departments is a matter Mr Kearney is curious about, and he specifically wants to know if a briefing report is given to the incoming secretary.

Mr Sterling, who left DETI in 2014, says key issues are identified and divided into "things for immediate attention, things the new permanent secretary needs to know and things that maybe could be read in slower time".

In response, the Sinn Féin MLA asks: "Did the RHI feature in that schematic that you've just outlined?"

Mr Sterling says he does not know whether RHI was "flagged up" in that document, although he accepts that he did read the handover.

"I could draw a supposition from that that RHI didn't feature, or at least it didn't feature with any concerns being flagged," Mr Kearney says.

Mr Kearney says he "gets" that the then permanent secretary didn't know what was happening.

"What I don't get is why your reporting line wasn't working," the Sinn Féin MLA.

Mr Sterling thinks carefully about his reply: "I think that type of question can only be answered through the fact-finding review that is being put in place."

The DUP's Trevor Clarke (below) says he senses that Mr Sterling could be trying to absolve himself of responsibility for the RHI scheme, but that suggestion is denied by the civil servant.

"I am not seeking to pass the buck to others and I will accept full responsibility for any failings which occurred during my time," he says.

Sinn Féin's Declan Kearney says he shared Mr Clarke's impression.

In listening to David Sterling's answers, he says "we were beginning to get a pattern of it was nothing to do with yourself".

Alex Easton of the DUP asks why proposed periodic reviews of some of the scheme's matters, such as tariffs, did not take place.

"I can't satisfactorily answer that," the former DETI permanent secretary replies.

But Mr Sterling adds: "I do regard the absence of a major review as being critical in all of this."

Mr Swann asks Mr Sterling whether, given the estimation of a potential £1.18bn loss, he accepts the RHI scheme represents "an unprecedented level of mismanagement".

Mr Sterling says the executive is facing a shortfall of £20m a year for 20 years as a result of the scheme, which he accepts is a "major loss" that is "unprecedented".

But he insists: "I don't think we in the Department of Finance or in the Department for the Economy accept that that has to be the outcome."

He says "everything has to be done to reduce the budgetary pressure going forward", suggesting that money from the scheme could be clawed back in some way and the projected cost could be reduced.

"We have to stop the exposure to the block [grant given to the executive by the Treasury]," Mr Sterling says.

PAC chair Robin Swann begins today's session by asking David Sterling what his role was in the RHI scheme.

Mr Sterling says that when he left DETI in 2014 he thought that "this is a scheme which isn't really working".

He says it was a scheme that mirrored one in Great Britain - "our counter-factual was kerosene, domestic heating oil, rather than natural gas".

"I wasn't closely involved in the development of the scheme," he adds.

The Renewable Heat Incentive scheme was introduced in 2012 by the Department of Enterprise, Trade and Investment (DETI), with the aim of increasing the uptake of renewable heat technologies.

But in July this year, the Northern Ireland Audit Office stated in a report, external that "serious systematic failings" in the scheme are likely to cost the Northern Ireland budget "hundreds of millions of pounds".

Economy Minister Simon Hamilton described the finding as "deeply shocking".

The Audit Office investigation was prompted when a whistleblower contacted the Northern Ireland Executive in January alleging that the scheme was being abused.

Image source, Tchara

One of the claims was that a farmer was aiming to collect about £1m over 20 years from the scheme for heating an empty shed.

At an initial PAC inquiry session, a senior civil servant apologised for the department's lack of oversight in the scheme and said that by the end of it, it will have cost the public purse a grand total of £1.18bn.

Last month, officials from Ofgem E-Serve, the scheme's administrators, gave evidence to the committee.

They came in for heavy criticism from MLAs, with the SDLP's Daniel McCrossan saying the scandal was "one of the biggest" that politicians had encountered since devolution.

We're back, and we'll shortly have this afternoon's meeting of the Public Accounts Committee (PAC), as it continues its inquiry into the catastrophic Renewable Heat Incentive scheme.

Image source, Thinkstock

In the hotseat today will be David Sterling, who was the permanent secretary at the Department of Enterprise, Trade and Investment when the scheme was set up.

Mr Sterling is now the top civil servant at the Department of Finance.

Shane Harrison
BBC NI Dublin correspondent

Former Irish Prime Minister Bertie Ahern may be about to rejoin the party he resigned from after a report into alleged planning corruption.

Image source, PA

Mr Ahern was a member of Fianna Fáil from the late 1960s until 2012.

At a meeting on Tuesday night, a motion was passed inviting Mr Ahern to become a member again.

The Executive Office Committee meeting is adjourned by chair Mike Nesbitt.

Join us again at 14:25 when the Public Accounts Committee continues its inquiry into the costly failure of the Renewable Heat Incentive scheme.

Mr McGuinness says that, after a "disastrous" interview with the Guardian, there was a general understanding that Mr Stitt had decided to resign but he had subsequently changed his mind.

"I think that he should reflect on that," the deputy first minister adds.

He says that would ensure that good work done by Charter NI in east Belfast "should not be overshadowed by the ridiculous comments [Mr Stitt] made".

The session finally turns to the elephant in the room - the controversy over the position of UDA leader Dee Stitt as chief executive of the government-funded community development group Charter NI.

Mr Nesbitt reminds the witnesses of Mr McGuinness's call for Mr Stitt to "stand down" from his role with the body.

He says that Mrs Foster subsequently told a newspaper that she welcomed the news that Mr Stitt was standing down "but he hadn't".

"That's what was reported to me," says the first minister, adding that she understood that Charter NI "have taken disciplinary action" following some of Mr Stitt's comments to the media.

Mrs Foster says it is not for her "to interfere with the employability of someone in an organisation", adding that to do so would be "illegal".

Referring to organised crime carried out by some gangs, the DUP's William Irwin says his Newry and Armagh constituency is known as the "fuel laundering capital in Northern Ireland".

Mr McGuinness says several agencies have recorded a "dramatic decrease in fuel laundering" as a result of new technology that has been deployed to tackle it.

Image source, PA

He points out that those who launder fuel "do huge environmental damage" and are "preying on the community".

"These people are only interested in making money," Mrs Foster says, and she commends the "difficult, dangerous work" carried out by the police and HMRC in tracking down those responsible for the crime.

Mr McGuinness says there is "no excuse whatsoever in our society for armed groups".

He rejects the notion of any political aspect to the activities of such groups.

"If there are people out there breaking the law they are criminal gangs," he says, regardless of whether they are claiming to be republicans or loyalists.