It's fair to say that Stephen Brimstone has had a challenging couple of days in the hotseat at the RHI Inquiry.

That isn't the end of the scrutiny for the DUP, though.

Image source, AFP

There'll be another big day at the inquiry tomorrow when Timothy Johnston - the DUP's chief executive and a former adviser to three first ministers - will be in the spotlight.

If you want some bedtime reading, his statement and that of the former DUP leader Peter Robinson have been published on the inquiry's website, external.

Join us at 09:45 tomorrow - for now, though, have a great evening...

Jayne McCormack
BBC News NI politics reporter

Two anonymous complaints were made that a DUP adviser was fraudulently claiming money through RHI scheme, the inquiry heard.

It was claimed that Stephen Brimstone was breaking the rules by using a biomass boiler to "heat his mansion house at the taxpayer's expense".

Image source, Pacemaker

He had installed the boiler in August 2015 and later became an adviser to Peter Robinson and Arlene Foster during their spells as Stormont's first minister.

But an investigation was carried out and Mr Brimstone was found to be complying with the scheme's rules.

Stephen Brimstone "wouldn't have went anywhere near" the RHI scheme either personally or professionally if he had to live the past few years all over again.

He says that everyone will make mistakes but "no-one wants to see their name on the front page of a paper, getting accused of ripping something off".

Image source, RHI Inquiry

Inquiry barrister Joseph Aiken acknowledges that Mr Brimstone's time before the inquiry has been "very difficult".

And Sir Patrick Coghlin tells the witness that while there was "pressure" on him during his time giving evidence he should "fully recognise the justification of it".

But he adds: "I think that anyone who attends to give evidence is deserving of considerable credit."

The belief that ministerial advisers are "a band of individuals just going and doing as we please... couldn't be further from the truth," insists Stephen Brimstone.

Asked what can be done to restore public confidence in them, he says there should be greater transparency about how they work.

Image source, RHI Inquiry

Inquiry chair Sir Patick Coghlin says that "one of the first things that needs to be looked at" if devolution returns in Northern Ireland are the rules on appointing and disciplining ministerial advisers.

Mr Brimstone agrees "without doubt" with inquiry panellist Dr Keith MacLean's suggestion that adherence with the existing rules "has been lacking" and a tightening up of the procedures is necessary.

The inquiry's questioning of Stephen Brimstone is coming to an end by touching on a part of his career that he'd probably prefer to forget.

He first entered the public eye in 2013 when working as an adviser to the then social development minister Nelson McCausland (below).

A BBC Spotlight programme reported that in 2011 Mr Brimstone had phoned the then DUP councillor Jenny Palmer to ask her to change a vote she was planning to make at a Housing Executive board meeting - she claimed he "bullied" her.

The programme ultimately resulted in the department's top civil servant ordering a fact-finding investigation to examine whether Mr Brimstone should face a disciplinary procedure.

Image source, Pacemaker

The fact-finding report recommended a disciplinary investigation but Mr McCausland decided that no further action need be taken against his adviser.

Inquiry barrister Joseph Aiken uses the case to take a wider look at the disciplinary process for ministerial advisers.

He asks Mr Brimstone if it seems right that the minister who appoints an adviser should also be the person who rules on their conduct.

"It's a difficult one for me to answer," says Mr Brimstone.

Stephen Brimstone says he wasn't part of what's been claimed was a core team of DUP advisers who were guaranteed a highly-paid job with a minister.

Former party aide Tim Cairns (below) told the inquiry he was assured by senior DUP adviser Timothy Johnston that he'd be kept on the "core" team of advisers if he agreed to continue working with the then enterprise minister Jonathan Bell after they'd had a row in the summer of 2015.

Image source, RHI Inquiry

He agreed to do that but he wasn't retained as an adviser after the next election.

Asked for his view, Mr Brimstone says he believed his job was "going to be placed in the balance" every time there was a ministerial reshuffle or an election.

"I certainly didn't view myself as being in any protected band, if such a thing existed."

Stephen Brimstone credits top DUP adviser Timothy Johnston with bringing expertise to Stormont's political institutions over the years that allowed a "period of stable government".

He says "time will tell" how big his friend and former colleague's "specialist" contribution has been.

Image source, PA

"I don't think anyone really realises what individuals had to go through at times in this country in order to bring a period of stable government."

Northern Ireland is "not a simple place to do government", he adds, explaining that significant "brokerage" has to be done behind the scenes between the DUP and Sinn Féin over even the "smallest document".

Sir Patrick Coghlin believes the evidence speaks for itself: "I think its very difficult for anyone to maintain that [Timothy] Johnston did not occupy a particular status."

Inquiry panellist Dr Keith MacLean (below) notes that Mr Johnston appears to have carried out a line management role within the DUP, including approving holidays and sick pay, as part of a hierarchical structure.

Image source, RHI Inquiry

He finds it hard to understand the reluctance to acknowledge that Mr Johnston - who has since become the DUP's chief executive - was a very senior figure in the party.

"His advice to ministers would have been very much respected," says Mr Brimstone.

DUP ministerial advisers at Stormont worked as a "team", says former aide Stephen Brimstone.

Each adviser saw their colleagues "as equals" but messages from those serving the first minister had "additional weight" because they were working in the Northern Ireland Executive's top office.

Image source, Getty Images

Sir Patrick Coghlin says that amounts to a "hierarchy" and that a first minister's adviser "did have priority in whatever he or she was saying".

Tim Cairns, a former DUP adviser, told the inquiry that everyone within the party knew that Timothy Johnston - who served the first minister - was "at the top of the tree".

But Mr Johnston - who'll give evidence to the inquiry tomorrow - denies in his written evidence any suggestion that one adviser stood above any other.

In his written evidence to the inquiry, senior DUP adviser Timothy Johnston (below) says he told his fellow adviser Richard Bullick and Arlene Foster in October 2016 about the complaint against Stephen Brimstone's RHI installation.

The first minister and all of her DUP advisers therefore knew that Mr Brimstone had applied to the scheme and was the subject of an investigation.

Mr Johnston says he had a conversation with Mr Brimstone about his involvement with the scheme but can't remember the details - Mr Brimstone can't recall the conversation at all.

Stephen Brimstone was told by fellow DUP adviser John Robinson in October 2016 that a complaint had been received about his RHI-registered boiler.

He says he believed the investigation was at an end but that was not the case.

Three months later he reactivated his sheep flock number - required for identifying, tracing and trading animals - which he'd deactivated back in 2009.

Image source, Getty Images

He says that in the eight-year spell when the number was inactive the shed that contained his boiler was used for his father-in-law's sheep.

Asked why he reactivated the number, he says his young sons were "extremely interested in sheep" and wanted to farm their own.

Sir Patrick Coghlin finds it interesting that Mr Brimstone reactivated his stock number "after all this fuss arose" with the complaint about his RHI installation.

The witness says there's no link between hearing about the complaint and the reactivation of his flock number: "I would've reactivated probably the next day if there had been a connection."

Arlene Foster and Martin McGuinness, along with senior officials and advisers, met at Stormont on 9 February 2016 to discuss several big issues - the RHI scheme topped the agenda.

The then first and deputy first ministers were briefed about its problems by Sir Malcolm McKibbin, the then head of the Northern Ireland Civil Service, and there was talk about its closure.

Image source, Getty Images

Stephen Brimstone - whose application to the scheme had been submitted by that point - was also at the meeting in his role as one of Mrs Foster's advisers.

He says he has no memory of the meeting and knows he should not have taken part in the discussion about the RHI.

Mrs Foster said on Tuesday that "it would've been better if he'd stepped out".

It appears the temperature is increasing in Stormont's Senate chamber...

Inquiry barrister Joseph Aiken begins the afternoon session by saying that "everybody has noticed the heat rising in here".

Image source, RHI Inquiry

Sir Patrick Coghlin jokes that it's "because of the quality of the debate".

Anyway, back to the action - the questions continue for today's witness Stephen Brimstone.

There'll be more questions for former DUP adviser Stephen Brimstone after everyone gets a bite to eat - join us again from 14:00.

Sir Patrick Coghlin says Stephen Brimstone's continued involvement in work on the RHI scheme in January 2016 raises questions about the "genuineness" of what he told Arlene Foster.

Image source, RHI Inquiry

The inquiry chair suggests to the witness that by continuing to allow DUP adviser colleagues to send him emails relating to the scheme, his declaration to the first minister that he'd applied to it was just done "for form".

Mr Brimstone shakes his head in disagreement.

Dame Una O'Brien asks Stephen Brimstone why - after he'd told Arlene Foster he'd applied to the RHI scheme - he remained involved email exchanges and discussions about it.

She asks: "Why couldn't he just have written a simple sentence? 'Guys, cut me out of this - I've already declared an interest to the first minister.'"

The witness admits that's what he should have done and adds: "I don't have an explanation as to why I didn't."

Dame Una says his continued involvement "effectively negated" his action in declaring his interest in the scheme to Mrs Foster.

In January 2016, Stephen Brimstone was continuing to receive documents relating to the RHI scheme - this time about its impending closure.

He claims he didn't respond to them and deleted them without reading them because it "wasn't a subject matter" that he "had any responsibility for".

Image source, RHI Inquiry

But Sir Patrick Coghlin tells him: "You know yourself that can't be a valid answer."

The inquiry chair says it's "difficult" for the panel to understand why he would've deleted RHI-related emails without reading them after he'd applied to the scheme.

Stephen Brimstone told Arlene Foster that he had a biomass boiler registered on the RHI scheme shortly after her appointment as first minister in January 2016.

He'd been appointed as one of her advisers and he remembers there was discussion about the potential closure of the scheme at the time.

Image source, PA

He says he felt the need to "make her aware that I had applied to the scheme" - she thanked him for telling her and "that was the end of the matter".

Asked why it wasn't put in writing, he says that "wasn't the way we worked" and things were simply sorted out in conversations.

"I think the inquiry has worked that out," smiles Joseph Aiken, referencing the many missing meeting minutes throughout the lifetime of the RHI debacle.

The document that Stephen Brimstone received outlined that the subsidies for the RHI scheme were going to be cut in the autumn of 2015.

He denies sharing the paper with anyone else.

Image source, Getty Images

At the time, his father-in-law was in the process of having a biomass boiler installed through the scheme and it was completed before the cap on the subsidies came into effect.

Mr Brimstone says he's never shared ministerial papers with anyone outside the Stormont executive.

Inquiry barrister Joseph Aiken says Stephen Brimstone had multiple opportunities to step back from his involvement in the work on the RHI scheme in July 2015.

The handling of the document he received from a fellow DUP adviser has been one of the central concerns of the inquiry.

The paper outlined the plan for adding cost controls and the subject line in the email sent to Mr Brimstone referred to the urgency required "to manage RHI expenditure".

Image source, Getty Images

He accepts that when he read the subject line he should not have opened the attachments and should've instead told his colleagues that he'd applied to the scheme.

He says he would've tried to keep his personal and professional circumstances apart in advising on the paper and would've looked at it in good faith.

But he knows that the "reality" is "you can't separate out your personal circumstances and the role you're playing".