Well, that turned in to a dramatic afternoon session, with Mr Stewart making some significant statements about the behaviour of DUP advisers in relation to the RHI scheme.

He's back to tell us more tomorrow morning - join us rom 09:45.

Goodbye for now!

BBC News NI

A top civil servant told the inquiry that he felt officials were treated as the "opposition" by DUP advisers as they tried to curb the rising cost of the flawed RHI scheme in 2015.

Chris Stewart said he felt the civil servants, advisers and their ministers were not "on the same team" as they struggled to rein in spending.

Mr Stewart, one of the top tier of management in Stormont's economy department, acknowledged that it was a big statement to make.

And he said had there been greater trust between officials, advisers and the minister they could have thrashed out an approach that might have been much more effective.

DETI decided to use a cost control system that would allow the subsidies on offer through the RHI scheme to be reduced once a claimant had reached a set usage limit for their biomass boiler.

That was chosen, rather than creating on a power to suspend the entire initiative.

Mr Stewart says that if the department had taken a more cautious approach and had been "more sensitive to the risk of a catastrophic spike" in applications then it would have taken the fail-safe option of the suspension power.

It's not unusual for ministerial advisers to consult with party colleagues about policies, says Mr Stewart.

But that process is "open to question" when the minister is not involved in the discussion.

Image source, RHI Inquiry

In the case of the cost control plan for the RHI scheme in the summer of 2015, he doesn't believe that Mr Bell was consulted.

"The discussions beyond Timothy Cairns were taking place with others in the party but I've seen no evidence that Jonathan Bell was involved at that stage."

Civil servants felt like they were "being treated as the opposition" by DUP ministers and advisers when they were trying to draw up plans to control spending on the RHI scheme in the summer of 2015, says Mr Stewart.

He acknowledges that it's a "profound thing to say".

Image source, RHI Inquiry

But he says out that advisers had negotiated with civil servants for the "minimum possible action" to control the cost of the scheme and then returned with new proposals that were "clearly designed to keep... expenditure... higher than we had intended".

"That does suggest to me that we weren't on the same team," he adds, sayig that there needs to be "trust and confidence between ministers, officials and spads" on policy work.

Relations between the then DETI minister Jonathan Bell (below) and his adviser were "at times strained" in 2015, observes Mr Stewart in his written evidence.

That appeared to be caused by Mr Bell's resentment of the way his adviser Timothy Cairns performed his party liaison role.

The most obvious example was the episode on a working trip to London when the pair had a row in an Indian restaurant that continued at a working breakfast the next morning.

Mr Stewart says he believes the minister may have misinterpreted advice he received on collective responsibility and the altogether different phenomenon of the relationship between the minster and his/her party leader.

There are "few if any instances" of civil servants challenging ministerial advisers - known as spads - about their activities, says Mr Stewart.

That's because they "do tend to operate independently of officials" and are accountable to their minister.

Image source, Getty Images

"There is no scope to direct a spad," he adds.

"They do tend to be the source of decisions emanating from the minister," he tells the inquiry and it's the advisers who "convey instructions or requests to officials" rather than ministers.

Mr Stewart says some people may have the impression that DETI officials "went into the office one day and suddenly released a huge amount of information on an otherwise unsuspecting world".

But the evidence, he says, shows a long chain of communication that began on the right side of the line but ended up on the wrong side.

The witness says it was a case of "their own words echoing back to them", with industry insiders seeking confirmation from the department.

"All of that led to a cascade of information that very rapidly was across the entire industry," he concludes.

Mr Stewart says he believed that inside information about changes to the RHI scheme was being leaked to the energy industry by ministerial advisers.

He tells the inquiry that the advisers have "connections with an industry" and "tend to be engaged in lots of communication".

One Democratic Unionist Party (DUP) adviser - Dr Andrew Crawford, who worked with the former DETI minister Arlene Foster - appeared before the inquiry last month and denied providing sensitive information about the scheme to the poultry giant Moy Park.

They "don't always recognise the same boundaries" that apply to civil servants in terms of what information should be shared, he says, basing that on "behavior that I observe".

But he admits that it's "supposition" and he "can't point to an evidence trail that would stand up in a court".

And asked by the inquiry barrister if its behaviour that's "more prevalent among the advisers of one particular party than others", Mr Stewart replies: "I would have no reason to think that."

A line was "very clearly" crossed by civil servants when they gave advance notice of crucial changes to the RHI scheme to people in the energy and agriculture sectors, says Mr Stewart.

They did that before the DETI minister had approved or even seen the plans, which were to make the scheme less lucrative.

Firms passed the information to clients who then signed up to the scheme before the amendments came into effect, causing a huge spike in applications that led to the bursting of the scheme's budget.

Image source, RHI Inquiry

One of the DETI officials responsible was Stuart Wightman (above), who's told the inquiry that the information was given to the firms as a "courtesy" - he's admitted that it was a "naive" thing to have done.

Mr Stewart says it was an "unwise thing to do" but he can't point to a clear breach of any civil service code.

"I think it was done with a good motive and an honest belief that it was a course of action they were entitled to take," he says.

Mr Stewart says he would not have chosen the RHI scheme as a way of incentivising people to use renewable heat.

It provided claimants with continued subsidies over 20 years and Mr Stewart says the risks of that model were not properly understood.

Instead, he says he would've picked an up-front grants scheme, which was an option that DETI had briefly considered but soon ruled out.

It would have been much easier to understand and control, he adds.

And given that the subsidy scheme became the preferred option, he says: "I can't see any rationale for not having cost controls in from the beginning."

It was a "day of complete dismay" when it dawned on civil servants that the overspend for the RHI scheme would have to be covered from DETI's own budget, says Mr Stewart.

That day came in December 2015, when the Treasury confirmed that it wouldn't be picking up the bill, which at one point was said to be as much as £700m.

Image source, PA

The officials realised that it would mean money earmarked for other projects would have to be diverted towards the scheme instead.

It was a "huge opportunity cost" and had "serious consequences" for the Northern Ireland Executive, he adds.

It's a "deplorable situation" that no-one at DETI sought clarity from the Treasury about the funding arrangement for the RHI scheme at a time when the budget was running out of control, says Sir Patrick Coghlin.

Virtually all of the civil servants who've appeared at the inquiry have said the funding mechanism for the scheme was unusual and one they'd never encountered before.

Basically, the Treasury would provide the money but it would not cover any overspend.

Image source, PA

If the scheme went over budget, DETI would have to cover that cost and there would effectively be a penalty imposed of about 5% in the case of an overspend.

But it wasn't understood by civil servants in Northern Ireland and in 2015 they debated among themselves interpretations of what it all meant instead of asking the Treasury for a definitive answer.

Mr Stewart acknowledges that "debates about what it might mean are pointless".

Relations between DETI's energy division - which was running the RHI scheme - and the finance division was "not always harmonious", says Mr Stewart in his witness statement.

He also says there was a "very sharp" exchange between the directors of the two divisions at one point when the spending problems became clear.

Image source, Getty Images

In March 2015, the energy division had asked finance officials for clarity on the budget for the scheme but it took months for that to be provided.

Mr Scoffield asks the witness if he felt there was a "difficulty" in the working relationship between the divisions that could've had an impact on how the scheme was being managed.

But Mr Stewart says that he didn't believe there was an "absence of relationships that was impeding business".

It wasn't until 19 May 2015 when Mr Stewart was first informed of problems with the RHI scheme, the inquiry hears.

Stuart Wightman, who was managing the running of it, emailed his boss John Mills to say that DETI's finance branch wanted him to curtail RHI spending to keep it within the budget.

He said the only way to do that would be to close the scheme immediately as it was already £4m over budget but his view was that RHI was a success and DETI should ask the Treasury for more funding.

Image source, Getty

Mr Stewart was copied in on the email - his response to Mr Wightman and Mr Mills was: "Keep me posted and pursue urgently with finance colleagues, escalating if necessary."

The inquiry barrister David Scoffield QC asks if the issue should've been drawn to his attention before that.

The witness says that what the officials did wasn't an unreasonable approach as the problem was understood to be a matter of budget uncertainty.

Conor Macauley
BBC News NI agriculture and environment correspondent

Busy opening session of the day - join us for more from 14:05.

It's a good idea for civil servants to get out from behind a desk to see what happens in practice, says inquiry panellist Dame Una O'Brien.

Mr Stewart accepts that but he points out that when DETI approached stakeholders "some of the information they received back was perhaps not as comprehensive or as accurate as it might have been".

He also has criticism for Stormont's agriculture department.

Image source, Getty Images

When DETI officials approached it for a briefing on the poultry industry the resulting report contained considerable input from poultry farmer and Ulster Farmers' Union representative Tom Forgrave.

"I'm as disappointed as they were to find that rather than getting independent advice and input [from the agriculture department] what they were actually getting was the views of the Ulster Farmers' Union by proxy."

Sir Patrick says that's a "fair point to make".

Civil servants working on the RHI scheme "were unworldly and naive" in their approach to dealing with the commercial sector, says Mr Stewart.

He even suggests - only half-joking, we think - that Ms O'Hagan, the so-called whistleblower, could be brought in to train civil servants.

Image source, RHI Inquiry

His comment comes after inquiry panel member Dr Keith MacLean (above) notes the lack of business experience of the part of DETI officials.

Dr MacLean reminds that inquiry that renewable energy firms had caught on to how lucrative the scheme was within days of it opening in November 2012 but it took civil servants years to notice, which he says is "quite mind-blowing".

Mr Stewart says that one way to remedy the lack of business sense within the department would be to employ specialists: "We do need to look at our talent management."

DETI has had to learn "very painful lessons" after ignoring constant warnings about the potential for abuse of the RHI scheme, admits Mr Stewart.

Concerns were raised by businesswoman Janette O'Hagan, who told the department that a flaw in the scheme meant some participants were burning fuel simply to earn money.

She first contacted the then enterprise minister Arlene Foster in 2013 and continued to raise the issue with DETI until 2015 but her warnings went unheeded.

Image source, RHI Inquiry

Mr Stewart says those warnings "should not have been dismissed lightly".

He tells the inquiry that she was "very determined in what she was saying" and it should've "aroused interest that resulted in further scrutiny and further examination" by civil servants.

And he adds that civil servants should presume that people presenting concerns are telling the truth.