There appears to be a fair bit of question still to get though for Ms Hepper, but Sir Patrick calls an end to the day.

We'll be back tomorrow morning from 09:45 for the penultimate day of inquiry hearings before Christmas.

Join us then!

BBC News Northern Ireland

A senior civil servant conceded that missing a basic sum that showed the fundamental flaw in the RHI scheme does "not look too clever".

Image source, Tchara

Fiona Hepper, who headed DETI's energy division, which set up the scheme, accepted that her team did not spot that the subsidy rate on offer was too generous.

That allowed claimants to earn more money the more fuel they burned, effectively collecting a profit.

Approval was granted by Stormont's finance department for the RHI scheme until the end of March 2015, after which point it would need to be reapproved, and a review of the initiative should also be carried out.

But as we know now, the review didn't happen and reapproval was never sought.

Image source, RHI Inquiry

Asked what she did to make sure those things would be done, Ms Hepper says she told John Mills, her successor as the DETI energy team boss, about it when she was handing the job over to him.

But Sir Patrick says there are "important dates and important events that must take place" doesn't appear to be "any easy way" for anyone new to the job "to find what they were supposed to be doing".

Mr Scoffield quotes from the scheme's business case that "under the RHI only useful heat is deemed eligible".

And that "excludes deliberately wasting or dumping heat for the sole purpose of claiming incentive payouts".

Image source, RHI Inquiry

He asks Ms Hepper why that was included in the case and what was done to ensure that the policy was adhered to.

She says there were checks carried out by the administrators Ofgem as they processed the applications and the payments, and monitoring was carried out by DETI.

The inquiry chair says he still has "difficulty" in seeing how everyone appears to have missed the glaring fact that the subsidy on offer through the scheme was higher than the cost of the fuel.

That error meant claimants on the scheme were overcompensated, as he explains: "The government is paying people more than they need for fuel."

Image source, Getty Images

Ms Hepper says she and her officials weren't "looking at it from that angle" and CEPA had fully explained why the subsidy was set at that level.

Sir Patrick asks whether DETI's view that CEPA's consultants were experts "inhibited you from the final common sense step" of doing assessments of their own.

"We had bought in this expertise that we didn't have and it was an error," says Ms Hepper.

Returning to the March 2012 submission to Mrs Foster (below), Ms Hepper agrees it was the next key point for the minister in relation to the RHI scheme since June 2011.

Mr Scoffield asks if that would have been an opportunity to inform her of the cost of the two scheme options, or that the price of the favoured model had increased following recent work by the consultants at CEPA.

Image source, PA

"That's not included in the submission is it?" he asks.

"No, it's not included," Ms Hepper says, adding that it would have been useful to Mrs Foster.

A document listing the risks of the RHI scheme was drawn up and Ms Hepper says it was monitored on a quarterly basis.

Inquiry chair Sir Patrick Coghlin asks: "Where would I find the record for that monitoring?"

Image source, Getty Images

Ms Hepper insists it was discussed but admits she never signed it off afterwards.

Sir Patrick appears unimpressed and questions why there's no record of the risk register being looked at, saying: "There must've been a degree of leadership for this group."

A submission was made to the then DETI minister Arlene Foster on 16 March 2012, asking her to grant approval for the RHI scheme.

Mr Scoffield notes that the casework committee minutes "required that the minister should be told expressly the reasons" why the subsidy scheme scheme was favoured over the up-front grants fund.

Image source, RHI Inquiry

But that doesn't seem to be have been done in the submission, he adds.

Ms Hepper says the submission "gives more of a synopsis", and suggests that there may have been a separate submission with a formal business case sent to Mrs Foster and the DETI permanent secretary.

"We can perhaps have a look for that," says Mr Scoffield.

A commitment was given to the casework committee that a monitoring committee would be set up to oversee the RHI scheme's budget, and assess information from the administrators Ofgem on the numbers of installations.

But that did not happen.

Image source, Getty Images

Ms Hepper says that was because applications were only coming in at a rate of three or four a month.

"We could manage that within the team," she says, adding that DETI a committee wasn't deemed necessary because it would have comprised the same people and they were already reporting to the department's finance division.

"It seemed to work quite well," Ms Hepper says.

It was viewed at the DETI casework committee meeting that it was "very unlikely" that the subsidy tariff on offer through the RHI scheme would be too generous.

That detail is included in the minutes from the meeting.

Image source, Getty Images

But as we know, now, and as DETI could have known at the time, the tariff was much higher than the cost of the fuel people would be using in their biomass boilers, meaning they could make a profit from it.

Mrs Hepper says the observation that the tariff was unlikely to be too generous was made on the assumption that CEPA's work in calculating it had been correct.

But she also says that stakeholders had told DETI that they actually wanted a tariff that was even higher than the one that was on offer.

By the time the RHI scheme came before the casework committee, the cost difference between the two options had reached a staggering £329m.

The rejected grants fund had a projected net cost of £24m, while the ongoing subsidy scheme, which was ultimately chosen, was £353m.

Image source, RHI Inquiry

Inquiry panellist Dr MacLean wants to know how big the cost difference would've had to have been for DETI to determine that that factor outweighed the other advantages it saw in the other option.

Ms Hepper doesn't know but says it was viewed by DETI as important to run a similar scheme as the Great Britain RHI, which was also a subsidy scheme

DETI saw a risk in the up-front grants fund model in that people who used the money to install a biomass boiler could stop using it when, for instance, gas became available in their area.

Ms Hepper says that was seen as "a significant enough risk" and could've made it difficult for Northern Ireland to meet its 10% target of renewable heat production by 2020.

Sir Patrick observes that it would be "a rather unusual thing to do" once someone had got the grant and fitted a biomass boiler to take it out if the fuel price rose by 5% to 10%.

"Has it ever happened?" he asks.

"I think it was just a view that it could happen, or a business could go out of business," Ms Hepper replies.

Dr MacLean points out that if a company goes out of business it doesn't continue getting the RHI subsidy either.

In the short term, admins costs for a subsidy scheme were estimated to be cheaper that a grants initiative.

Mr Scoffield points out that it is noted in the casework committee meeting minutes that it was discussed that a subsidy scheme could be run at a "fraction of the cost" of the grants offer.

Image source, CEPA

But as the table above shows, a subsidy scheme - listed under "NI RHI - Alt" - was in fact projected by CEPA to be £2.5m more expensive over than a grants scheme - listed under "Challenge fund" - its 20-year lifetime under Funding 2 model that was ultimately chosen.

Ms Hepper says she can't remember who in the meeting made the comment that suggested otherwise.

Resuming his questioning of DETI's former energy division boss Ms Hepper, Mr Scoffield looks at the options considered by the casework committee and the reasons why an ongoing subsidy scheme was preferred over the up-front grants model, knows as a challenge fund.

"The question really is whether the non-monetary considerations outweigh the fact that the challenge fund provides the most renewable heat at the lowest cost," he explains.

Image source, RHI Committee

These include the affordability of administration.

The estimate of the cost of administering a challenge fund was based on the experience of Reconnect, a previous DETI energy scheme, and Ms Hepper says it was "the best comparator we had".

Mr Scoffield questions whether Reconnect provided a useful comparison given that "well-over half" of the figure for administration was spent on publicity.

Mr Scoffield says that feels like a natural point at which to break for lunch, so we'll be back at 14:00 for the afternoon session.

We'll leave you for now with a little snippet of some of the splendid seasonal singing that greeted us in Stormont's Great Hall this morning!

That massive cost difference of more than £200m between the two options - the cheaper up-front grants fund and an ongoing subsidy offer, which was eventually chosen for the RHI scheme - is touched on again...

Mr Scoffield points out that the "stark difference" isn't mentioned in the casework committee meeting minutes and he asks whether it was mentioned at all.

Ms Hepper says she assumes that the tables outlining the differences in the options were discussed, but she "can't recall the detail of a meeting that was all those many years ago".

There was an option for DETI to carry out an emergency review of the RHI scheme, according to the minutes from the casework committee meeting.

But there was no express provision for that included in the regulations.

Image source, Getty Images

Ms Hepper says that's because as far as DETI was concerned it could review the scheme whenever it liked, and didn't need legislative approval: "It was our policy, it was our scheme - we could do that."

It is pointed out that a previous renewables scheme in Northern Ireland did have an explicit provision for emergency reviews written into its regulations, and that gave participants clarity and confidence about when changes might be made to it.

Ms Hepper says it's "hard to say with hindsight" whether writing such provisions into the RHI scheme would've been a good idea.

Mr Scoffield wants to know if the casework committee thought it was giving approval to a scheme with a budget of £25m over four years, or the "full estimated subsidy spend" of upwards of £400m over 20 years.

Image source, RHI Inquiry

The committee members will answer for themselves, says Ms Hepper.

But she adds: "My view would have been that it was absolutely the £25m but that this came with a tail and that had to be taken note of."

Mr Scoffield asks Ms Hepper if she had been aware of the problems that occurred in 2010-11 in Great Britain with the Feed-In Tariff (FIT) electricity generation scheme.

In particular, there had been "spikes" in uptake in that initiative.

Image source, RHI Inquiry

So, did she appreciate her planned emergency stop mechanism for dealing with a similar problem with the RHI scheme "might be insufficiently speedy"?

"At that time we didn't envisage that happening," Ms Hepper replies.

Conor Macauley
BBC News NI Agriculture and Environment Correspondent