It's been a long afternoon, and Mr Cockburn has another evidence session to look forward to in the morning.

Mr Lunny has requested an early start, so we'll be back in the Senate chamber at 09:45 GMT with more in-depth coverage of the inquiry.

We'll leave you with a snap of the lovely Christmas tree in the Great Hall at Stormont.

Goodnight for now!

It's pointed out that even without gaming, claimants could still be overcompensated by the RHI scheme.

Mr Lunny explains that if a claimant used their biomass boiler for a longer period each year than was assumed in the calculations used for setting the tariff, there was a risk that they would collect more money than was intended.

He wants to know why CEPA didn't warn DETI about that possibility in any of its reports.

Mr Cockburn says the issue "everyone" was focused on was trying to meet the target of 10% of Northern Ireland's heat being produced through renewable sources by 2020.

Sir Patrick refers back to the overgenerous tariff, which as we know was clearly laid out in the CEPA report, and says it should not just have "jumped out at you, but been written all over you".

"That didn't happen in this case," he adds.

Tables provided with CEPA's 2012 addendum illustrate how the tariff is calculated contained mistakes, Mr Cockburn accepts.

Mr Lunny shows that CEPA emailed DETI a number of tariff spreadsheets two weeks after the receipt of the addendum, which were described as a draft that the consultants take "no responsibility for".

Image source, RI Inquiry

Mr Lunny says these could have been used to derive the correct figures.

He also notes that nowhere was it said that the tables at the back of the 2012 report "are flawed - they have several incorrect values in them".

"We didn't realise there was an error in the report," says Mr Cockburn.

Mr Cockburn says he "would never have thought anybody would have been able to game" the RHI scheme as much as some did.

Quick explainer - so-called gaming was one of the biggest issues the led to the initiative's catastrophic overspend, which could hit £700m without the introduction of cost controls.

Some businesses installed several smaller boilers rather than a single larger boiler in order to obtain a higher tariff that was on offer for smaller installations.

It also happened to be the case that the subsidies on offer were higher than the cost of the fuel used in biomass boilers installed through the scheme, allowing claimants to earn more cash the more fuel they burned - that's referred to as the "perverse incentive".

"There's gaming and then there's gaming," Mr Cockburn says, adding that those setting up the scheme never thought claimants would do it to the extent that actually happened.

Looking at figures from the CEPA addendum, Mr Lunny demonstrates that there are "three times in that relatively short report where we see 5.9p as the small commercial biomass tariff and a fuel cost that's below it".

He says these were three opportunities to spot that the tariff was noticeably higher than the input fuel cost.

Image source, RHI Inquiry

So, did Mr Cockburn know why it had been missed?

"It shouldn't have been. I think unless you were really familiar with the 2011 report, it was fresh in your mind, then you might have just missed it," the consultant replies.

Reasons why a warning for the need for a cap was missing from the addendum continue to be probed by the inquiry.

Mr Cockburn insists again that the budget DETI provided for the work didn't allow for more comprehensive work: "I know everybody thinks it's high - I don't think it is for that amount of work."

Inquiry panellist Dame Una O'Brien challenges that view, asking: "How much would it have cost to put that sentence or two on the end?"

Mr Cockburn acknowledges that it could have been included.

The inquiry is trying to get to the bottom of what work was to be done by CEPA on its addendum in 2012.

Mr Cockburn says the consultants would not have agreed to do a detailed review of their 2011 report, adding: "I think the initial assumption was that we would be writing some notes on it."

"Writing a note?" says Sir Patrick. "You'd just be writing a note?"

Next under the microscope is CEPA's addendum report from February 2011 (click to download), external on options for a renewable heat scheme.

The consultants were approached by DETI to reassess its work in the original report on the basis that new evidence on the sizes of boiler installations and costs of fuel had emerged, which would affect cost projections for the scheme.

Image source, Getty Images

A table in the report shows that as a result of the new evidence, the overall cost of subsidies in the RHI scheme until 2020 was projected to increase by £111m to £445m.

Mr Lunny points out again that, just as in the original report, the need for a cap on the scheme was not mentioned in CEPA's addendum.

Mr Cockburn says CEPA believed it was only being asked to carry out "limited additional analysis", and if the company had been assigned a bigger budget by DETI to conduct a full review the lack of a cap "would've been picked up".

Inquiry panellist Dr Keith MacLean intervenes to clear up a point from this morning regarding the daily rates that CEPA charged for its consultancy work.

He says Mr Lunny had referred to a fee of £750 an hour, "which appears perhaps a little bit optimistic".

Image source, RHI Inquiry

Mr Lunny apologises - he had meant to say £750 a day.

"That's almost getting up into lawyer levels," says Dr MacLean.

It transpires that the £750 was for just one of the consultants, and that DETI was also being charged £1,400 a day for the services of CEPA's Ian Morrow.

Mr Lunny turns to a series of 2017 emails in which Mr Cockburn discusses the CEPA report conclusions with his colleague Ian Morrow, who worked with him on the 2011 paper.

Mr Morrow says it was very clear that DETI favoured an RHI scheme, and says the department was "concerned about their ability to administer a challenge fund".

Mr Morrow goes on the say that CEPA agreed with DETI "that the report would not make a firm recommendation".

Image source, RHI Inquiry

He says DETI did not accept this easily: "They wanted us to make a recommendation but we said that it could only be to do a challenge fund, which they didn't want."

Mr Lunny puts it to Mr Cockburn that CEPA's conclusion "was being tempered in some way" to suit a DETI policy agenda.

"I have to say it was," Mr Cockburn replies, adding that he thinks there "probably was some negotiation" on the matter.

CEPA was put in a "really difficult position" by DETI after it decided not to take its advice that a grant scheme was the best value-for-money option, says Mr Cockburn.

"We could take the horse to water, but I don't think we could thrust its head into it," he says.

But panel member Dame Una O'Brien says CEPA's advice was "confused" and asks if the consultants gave what appears to be a non-committal answer in order to "satisfy the client".

Image source, RHI Inquiry

Mr Cockburn defends CEPA's role, saying he would "never write down something we didn't believe in".

Dame Una asks if CEPA felt under pressure from DETI to produce a conclusion that would justify a perhaps pre-determined decision to go with a subsidy scheme over a grant option.

While he doesn't directly answer that question, he suggests DETI may have found it easier to run with the subsidy scheme because it was similar to the existing Great Britain RHI initiative, rather than setting up its own wholly-new grant scheme.

Suitably refreshed we return to the Senate chamber at Stormont for this afternoon's session.

After examining CEPA's report conclusion before lunch, Mr Lunny confirms with Mr Cockburn that it identified the challenge fund as the best option on a value-for-money basis.

Image source, RHI Inquiry

Mr Lunny then turns to the witness statement of DETI's head of analytical services Shane Murphy, who says CEPA's report was one of "mixed or inconclusive messages".

He adds that CEPA's enthusiasm for the challenge fund is clear within the body of the report, but his impression is that the report's recommendations "were much less clear cut".

Mr Murphy says it kept "the door open" to either a grant fund or a subsidy scheme.

That's where we take a break for a bite of lunch.

Image source, PA

We'll have more from Messrs Lunny and Cockburn at 14:15 GMT - join us then.

Delving through tables in CEPA's 2011 report, Mr Lunny points out that an up-front grant scheme consistently outperformed an ongoing subsidy scheme in the cost forecasts.

In the summary at the end of its report, CEPA says that a grant scheme "appears to be more attractive" than a subsidy option, but "there are other pros and cons of these two options".

For example, while a grant scheme is cheaper over its lifetime, CEPA says it can be difficult to ensure that the renewable heat system purchased using the money is used after the grant is paid.

For a subsidy scheme, payments are only made for the production of renewable heat, effectively guaranteeing that the equipment will be used, but it is ultimately more expensive.

It would also be easier to administer because DETI could "piggy-back" on the system used in the similar GB RHI scheme, therefore saving costs.

CEPA concludes that the best value-for-money is offered by the grant scheme, but if the other benefits of a subsidy scheme outweigh that then a subsidy scheme "should be pursued" for the commercial sector.

Mr Cockburn is asked whether he feels the conclusion has been set out "in clear enough terms", to which he replies: "I don't think it's ambiguous".

Mr Cockburn says there are various emails going back as late as April 2011 where "we're still trying to persuade" DETI to consider a challenge fund option for its renewable heat scheme.

That option would be in the form of an up-front capital grant scheme for biomass boiler installations, with the grants awarded on a competitive basis.

He says CEPA felt that the challenge fund idea "was a little bit of a stalking horse".

Image source, Getty Images

There's "no doubt", he says, that "there was always a preference" at DETI to run an ongoing subsidy initiative instead, which was eventually adopted for the RHI scheme.

He speculates that DETI may have felt uncomfortable with a scheme that was too different to what the ongoing subsidy scheme that was on offer in Great Britain.

The apparent preference for a subsidy scheme will have to be examined when DETI officials and Mrs Foster appear before the inquiry, says Mr Lunny.

A submission to minister Arlene Foster in June 2011 with recommendations for the RHI scheme "does not reflect" what CEPA had concluded in its draft report for DETI, says Mr Cockburn.

Image source, Getty Images

In the submission, it is claimed that an ongoing subsidy scheme is the "preferred approach" and offers the "highest potential renewable heat output at the best value".

But the analysis that CEPA had carried out found that an up-front grant fund was actually a better option in that regard.

CEPA produced additional material in 2012 for DETI on aspects of the planned scheme, and Mr Lunny asks whether its staff read the previous report from 2011 and the subsequent public consultation before it began that work.

He suggests that would only have taken "an hour or two", but Mr Cockburn says that as far as far as he's aware, nobody did that.

Image source, RHI Inquiry

Mr Cockburn indicates that CEPA tried to limit the amount of work they would have to do on this piece of work "because there was a limited budget".

Sir Patrick puts it to Mr Cockburn that CEPA was engaged DETI "as professional experts with a professional reputation" but their arrangement "wound up with you not being keen to do any further work because you weren't paid enough".

Mr Cockburn refers to the lack of budget, saying: "We're a commercial organisation."

Inquiry chair Sir Patrick Coghlin says he finds it "strange" that Mr Cockburn's expert team at CEPA had to make assumptions about the funding available and the need for a cap for the scheme in their early work.

"Why not ask what was the situation if they go overbudget?" he asks, particularly given that it is in a "novel, demand-led" scheme.

Mr Lunny picks up the point and asks Mr Cockburn: "Was it safe to assume that anybody receiving your report would have such a thing as a working assumption in a relatively novel area like this?"

Mr Cockburn points out that CEPA expected that "a lot more detailed design work would follow on" from what it was doing.

Image source, RHI Inquiry

Would it not be common practice to preface an independent expert-led report to list a series of working assumptions, asks Mr Lunny, "to keep yourself right".

This would allow the recipient of the report to identify any incorrect assumptions from the start.

"It could have been caveated more," says Mr Cockburn. "I think the people we were working to directly knew the situation."

CEPA's work in assessing options for an renewable heat scheme was based on its understanding that DETI had been given £25m for it by the Treasury over four years, says Mr Cockburn.

He says he believed that the funding was "finite" and that sum could not be exceeded.

It was also his understanding, he says, that the scheme would be run on a first-come-first-served basis, and once the budget had been used up that would be the end of it.

Mr Lunny points out that nowhere in CEPA's work for DETI was "clear advice or clear warning given" that there would be a need for a cap on the scheme, if it was going to be an ongoing subsidy scheme.

Mr Cockburn says he "thought that would've been just common sense".

AEA's cooperation with CEPA is probed by Mr Lunny. What was AEA's role in the consultation work?

"Because they had the most experience of renewable heat, I think both DETI and the CEPA team would ask their ask their views on different things," says Mr Cockburn, "It was a collaborative arrangement".

Image source, RHI Inquiry

Mr Lunny gives the example of how a warning from AEA about the dangers of people "gaming" the RHI scheme "percolated up" to CEPA.

Gaming was the installation of multiple small boilers instead of one large, more efficient boilers in order to collect a higher tariff on offer.