The "politics of the situation" was that Mr Cairns took instructions from not only his own minister but also from the first minister's office through advisers, he says.

"The leader of any political party... obviously carries considerable influence and power over and above their ministers," he adds.

Image source, Getty Images

There were times when he was "invited to carry a message" from the first minister's office to his minister, Mr Bell.

It's "hard to conceive" of anyone involved - ministers, advisers of other DUP employees - who "when Peter Robinson asked them to do something felt that they wouldn't do it".

Mr Bell has told the inquiry that Mr Cairns wouldn't have been his first choice as an adviser when he was appointed as DETI minister and that he could see any number of people who were better suited to the role.

Image source, RHI Inquiry

Mr Cairns says he can't comment on that opinion and reiterates his point that he didn't have a say in his appointment.

The inquiry barrister Donal Lunny describes it as "an arranged marriage of inconvenience", prompting a few smiles inside the Senate chamber.

Mr Cairns claims that he felt in "fear of assault" from Mr Bell after he became "extremely angry" during some "general office banter" between DUP advisers about Valentine's Day in 2013.

Image source, Pacemaker

"His fists were clenched," he says and he says he complained about it to DUP adviser Timothy Johnston.

"It's fair to say... that other people had similar experiences - I'll leave that there," adds Mr Cairns.

Mr Bell wasn't a minister who was across his ministerial brief, claims Mr Cairns.

Asked why DUP advisers were so reluctant to work with Mr Bell, he says the relationship between the adviser and the minister can be very difficult "if the minister isn't absolutely across his brief".

Image source, RHI Inquiry

He says every adviser in the Office of the First and deputy First Minister - where Mr Bell had been a junior minister before his move to DETI - had experienced the problem with him, particularly when it came to meetings with outside bodies, businesses and in international relations.

Asked by inquiry panellist Dame Una O'Brien (above) if Mr Bell wasn't being fully briefed by his officials or didn't understand the briefs he was given, Mr Cairns says: "He wasn't reading it, in simple terms."

No DUP advisers wanted to work with Mr Bell, says Mr Cairns.

It "defied logic" for him to be appointed as the DETI adviser and he believed it was better for Dr Andrew Crawford (above) - who had been the adviser in that department for a few years - to remain there.

But Dr Crawford went with Arlene Foster when she moved from DETI to Stormont's finance department.

Mr Cairns was given five minutes' notice and "no chance to object" to his appointment as Mr Bell's adviser at DETI, he says in his witness statement.

He says he was told by the then DUP leader and first minister Peter Robinson (below) and didn't have a conversation about it with his new minister.

Image source, AFP

Mr Cairns believes he couldn't have objected to Mr Robinson's decision: "It would've been a very interesting lunchtime between us all if I'd refused to go.

"To test that would've been cataclysmic."

The DUP did not adhere to the correct procedure for appointing ministerial advisers, says Mr Cairns.

Asked what he did about it, Mr Cairns says he "didn't believe [it was] my place to ask the party officers to do that".

He believes the decisions about who to appoint as ministerial advisers would've been made by a "triumvirate" of the DUP leader and senior DUP advisers Timothy Johnston and Richard Bullick (above).

Mr Cairns says its correct that there was no open competition for special adviser jobs.

The code of practice for the appointment of advisers states that it is for ministers to appoint their own advisers.

Mr Cairns says he doesn't believe there's any conspiracy to "fit Mr Bell up" but he thought Mr Robinson should've been "exposed" for the way he protected the former enterprise minister.

And he accepts that he was happy if the DUP didn't want to include that detail in what it told the press about Mr Bell.

"And that's politics is it?" asks Sir Patrick.

Image source, Pacemaker

"That's politics I'm afraid - it's a grubby world," says the witness.

"That's something of an understatement," responds the inquiry chair.

Mr Cairns claims that at the time he was being contacted by journalists constantly at the time - December 2016 - and he was "looking for help" from the DUP to "take on some of the press burden" because he "just wanted to be out of the public eye."

Denying that there was an attempt to smear Mr Bell, the witness tells the inquiry: "I'm not sure if you're putting forward something that is true that can be characterised as a smear."

Asked about his text in which he stated that he'd "fit my story in with the party narrative", Mr Cairns says he meant that the former DUP leader Peter Robinson had tried to protect Mr Bell but he believed that party would take a different view.

Mr Cairns claims he raised allegations of bullying from Mr Bell with the former first minister but that Mr Robinson did nothing about it.

Image source, RHI Inquiry

"The huge question, I think, for Mr Robinson is if he did receive allegations... from other people about Mr Bell's behaviour... why did Mr Robinson appoint Mr Bell to the position he was in?" he says.

Sir Patrick (below) says the "clear inference" of Mr Cairns' evidence is that he was prepared to "modify whatever it was you said about Mr Bell to protect Mr Robinson".

He denies that but then accepts that he was prepared to go along with what Sir Patrick calls "the party narrative".

"That seems a rather unpleasant way to run a party," observes the chair.

Mr Bell claims that his former adviser's evidence to the RHI Inquiry is part of a "smear campaign" against him.

One of the things on which he bases his view is text messages that Mr Cairns sent to senior DUP advisers.

Image source, PA

The first, to Mr Johnston in December 2016, reads: "Please be assured of my support. I remember saying to Peter [Robinson] with you... that [Mr Bell] is a liability. I will do whatever is needed to expose lies."

Another, to Richard Bullick, reads: "My view is [Mr Bell] needs to be exposed... I've said to you, Arlene and [Mr Johnston] that I will fit my story in with the party narrative and what is best with [the] party."

Mr Bell referenced that second message in his evidence last week.

Mr Cairns briefly left his role as a DUP adviser to Mr Bell in October 2015.

He says he was given a commitment by Mr Johnston in October 2015 that if he returned to his role as an adviser to Mr Bell that it was likely he'd be kept on in the DUP "core" team of advisers after the Northern Ireland Assembly election the next spring.

Image source, RHI Inquiry

But that didn't happen and he perceived that to be a breach of their agreement.

"Timothy Johnston is Timothy Johnston...I know the way he operates," he says.

Asked about the end of his career as a DUP ministerial adviser, Mr Cairns says it became clear after the 2016 Northern Ireland Assembly election that he would not be keeping his job.

He says he wasn't given an explanation as to why he was not being kept on.

He had joined the DUP as a teenager, became involved with the Ulster Unionists at university and reengaged with the DUP after the Good Friday peace agreement.

He says he doesn't believe the loss of his adviser job has soured his relationship with party leader Arlene Foster or senior adviser Timothy Johnston (above left, with former DUP leader Peter Robinson), who is now the party chief executive.

Mr Johnston says in written evidence that Mr Cairns blamed him personally for the loss of his job and they had a "frank discussion" about this at a meeting in 2017.

"No-one, I believe, would be appointed as a special adviser within the DUP without Timothy Johnston's assent," says the witness.

Asked about his time working of Gavin Robinson's (below) Westminster election campaign in the spring of 2015, Mr Cairns says he did it "freely of my own volition".

In Mr Bell's witness statement, he says that Mr Cairns had been intimidated by fellow DUP adviser Timothy Johnston into running the campaign "or he wouldn't have a job... in the morning".

Image source, Pacemaker

Mr Cairns says he has "no idea" where Mr Bell got his version of events from.

The inquiry chairman Sir Patrick Coghlin asks the witness: "Why would he say that? You say that's not true so one of you is lying."

Mr Cairns says it "goes to the nature of the relationship that I had with Mr Bell".

Tim Cairns was Jonathan Bell's adviser at Stormont's Department of Enterprise, Trade and Investment (DETI) in 2015 and 2016 and offered guidance to the minister about the controversial RHI scheme.

His version of the RHI debacle is almost completely opposing to that of Mr Bell.

He has a colourful and varied background - he worked as a solicitor before going to Canada to study religion and he became a pastor for three years.

Image source, RHI Inquiry

On his return to Northern Ireland in 2011 he worked for the DUP as its head of policy before advising Mr Bell when he was a junior minister at Stormont.

He also ran Gavin Robinson's Westminster election campaign in 2015, after which he began working with Mr Bell at DETI.

His time as a DUP adviser ended in May 2016 and he now works as the Northern Ireland director for the Community Transport Association, a transport services charity.

Tim Cairns affirms that he will tell the truth to the inquiry.

He's submitted three witness statement to the inquiry, which offer some eyebrow-raising details - you can find them here, external, here, external and here, external.

Image source, RHI Inquiry

Junior counsel Donal Lunny is asking the questions today - it's the first time we've seem him at the inquiry since its last day before the summer break began in June.

Mr Lunny explains some of the topics he wants to cover with the witness, which include his role as a DUP adviser, the events concerning RHI in the summer of 2015 and the closure of the scheme.

Jayne McCormack
BBC News NI politics reporter

The minister in charge of Stormont's enterprise department during the period that the RHI scheme did the most damage to the public purse gave evidence to the inquiry across two dramatic days.

Jonathan Bell claimed on Thursday that a senior figure in Stormont's Executive Office was briefing against him as a "monster who had to be put to sleep" hen he was a DUP minister.

He said the individual was David Gordon, who headed the communications for the Executive Office in 2016.

Image source, PA

And on Friday he denied that he opposed a two-week extension to the closure of the RHI scheme because it made him look foolish.

The claim has been made by DUP leader Arlene Foster and two party advisers, who have said his primary concern was not about protecting public finances but that he didn't want to make a political u-turn that would have been personally embarrassing.

BBC News NI

It is the scandal that sent Northern Ireland's devolved government up in flames and risked leaving taxpayers hundreds of millions of pounds out of pocket.

The RHI debacle has exposed serious flaws in the region's civil service and governance systems.

BBC News NI has followed every minute of the inquiry into the scandal since it started last autumn.

Read our review of some of the major revelations that have emerged so far.

BBC News NI

An independent inquiry into the RHI scandal was established in January last year by the then finance minister Máirtín Ó Muilleoir.

He ordered it in the wake of the huge public concern and what was then a developing political crisis surrounding the scheme.

The RHI Inquiry began in November and Sir Patrick Coghlin (below), a retired Court of Appeal judge, is its chair and has been given full control over how it will operate.

Image source, Pacemaker

It will look at:

• the design and introduction of the RHI scheme
• the scheme's initial operation, administration, promotion and supervision
• the introduction of revised subsidies and a usage cap for new scheme claimants in 2015
• the scheme's closure

For more information on the RHI Inquiry, you can read our handy Q&A.

When the scale of the overspend emerged, public and political concern rocketed.

As the minister in charge of the Stormont department that set up the RHI scheme, the DUP leader Arlene Foster faced calls to resign from her role as Northern Ireland's first minister in December 2016.

Image source, PA

She resisted, and Sinn Féin's Martin McGuinness then quit as deputy first minister in protest at the DUP's handling of what had by then become a full-blown political crisis.

That move brought about the collapse of the Northern Ireland Executive. Now, well beyond a year-and-a-half on from that, Northern Ireland remains without a devolved administration.

You can find much more detail on the RHI scheme in our need-to-know guide.

The budget of the RHI scheme ran out of control because of critical flaws in the way it was set up.

Claimants could effectively earn more money the more fuel they burned because the subsidies on offer for renewable fuels were far greater than the cost of the fuels themselves.

At one point the estimate for the overspend was set at £700m if permanent cost controls weren't introduced - temporary cuts have since pulled the budget back on track for now.

Whatever the scale of the bill, it will have to be picked up by the Northern Ireland taxpayer.