Jonathan Bell with Niall Gibbons of Tourism Ireland in December 2015 - the month before it is alleged that at a meal with Mr Gibbons and others Mr Bell got drunk

Jonathan Bell, whose party was founded by the vehemently teetotal Ian Paisley who used to denounce “the Devil’s buttermilk”, presented a sober evangelical Christian image of himself to the public and spoke out against the dangers of alcohol.

But according to his former special adviser (Spad) Timothy Cairns, Mr Bell’s behaviour in private could be erratic, explosive, physically threatening and involved angrily swearing at his staff.

Last week Mr Bell denied that he had ever been physically threatening or acted inappropriately as minister and suggested Mr Cairns was altering the facts to discredit him.

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In detailed written statements to the RHI Inquiry, Mr Cairns set out his extraordinary account of a trade mission to the US in January 2016 – the point at which desperate efforts were being made to close the RHI scheme, which at that point was clearly going to hit Stormont’s budget.

He said: “On the Wednesday evening of our departmental trip to San Francisco and New York we had an evening off. Mr Bell, Mr Kerr and Mrs Fullerton [DETI civil servants] and myself went for dinner.

“Before dinner we each had a drink in the bar. Mr Kerr, Mrs Fullerton and myself carried what remained of our alcoholic drinks to the restaurant. Mr Bell had consumed his.

“At the table Mr Bell asked if anyone was having wine. Everyone stated they were OK.

“Mr Bell asked if he ordered a bottle of wine for himself, to take the bad look off it, would we get wine glasses. We agreed... Mr Bell then consumed the rest of the bottle on his own.”

Mr Cairns went on: “After dinner we went to a pub in New York. Mr Bell was clearly intoxicated. In the pub we each had a further alcoholic drink.

“Mr Bell fell asleep in the pub. The waitress informed us if Mr Bell did not wake up we would have to leave as he was clearly intoxicated. Mr Bell woke up, consumed his pint of Guinness and ordered another.

“We were reluctantly served, but were told if Mr Bell fell asleep we would be asked to leave. He again fell asleep and we were immediately asked to leave.

“Mr Bell was unsteady and had to be helped back to the hotel while he sang the Deep Blue Something hit single ‘Breakfast at Tiffany’s’ at full volume.”

The inquiry has already published written evidence from the most senior civil servant in Mr Bell’s department, Andrew McCormick, in relation to the impact of what he carefully described as “quite a late night” for the minister.

Dr McCormick said that the following morning “it was noted that Jonathan Bell was visibly tired at a key meeting” and “it was my impression that, in consequence the minister was unable to participate fully in the meeting in a constructive way, as I would have hoped”.

Mr Cairns set out a range of alleged behaviours by Mr Bell which he said made for a difficult situation with the department and within the DUP.

He said that on one occasion when Mr Bell overslept, meaning that he missed Assembly business, he furiously told Mr Cairns and his private secretary Sean Kerr: “You two boys have f****d up!”

Mr Cairns added: “When Mr Bell was acting in an aggressive manner he often swore. This is not something I would condemn anyone for as I would often swear myself, however I raised it with [fellow DUP Spad] Mr Bullick as Jonathan was attempting to portray an evangelical Christian image on the Nolan show. This was at odds with my experience.”

He said that Mr Bell “exhibits a changeable personality” and that while he “usually presented a confident manner...at other times Jonathan would be emotional, quoting the Bible and wondering about the eternal destiny of his soul”.

On those occasions, Mr Cairns said that their relationship would be good but that at other points Mr Bell “exhibited an explosive personality...during these periods our relationship was strained”.

He added: “My relationship with Jonathan Bell was good while I affirmed what he wanted to do. If I provided negative advice, he would exhibit an explosive personality.”

The minister “would be aggressive one minute and your friend the next”, Mr Cairns said, but added that “when he wasn’t acting in an aggressive manner he offered me a job in the new company he was setting up to establish trade links with China. He went as far as to offer me a salary of £50,000. This offer was made on the last day before Purdah. This highlights his erratic nature.”

He referred to Mr Bell having “pinned a woman against a wall”, something he said Mr Bell having “cornered [DUP MLA Michelle] McIlveen at the party conference and [he] had berated her for some time until she broke down in tears. It is also a reference to his behaviour toward [DUP MP Emma] Little-Pengelly”.

Mr Bell said that he believed that both woman had reported Mr Bell’s behaviour to the then DUP leader, Peter Robinson, “who took no action”.

Mr Cairns said that he says that Mr Robinson – who was close to Mr Bell, who employed at least one of the then DUP leader’s children in his constituency office – had been aware of “historic allegations made against Mr Bell and his known volatility”, but despite that “Mr Robinson proceeded to promote him. In my opinion this decision by Peter Robinson is inexplicable”.

Just two days after Mr Bell’s explosive interview with Stephen Nolan, Mr Cairns sent a message to Arlene Foster’s then Spad, Richard Bullick, saying that Mr Bell “needs to be exposed” but “I can’t expose my part without putting boot into PR [Peter Robinson].”

He went on: “Don’t get me wrong, I’m very happy to do that. But the more fuel we put on a fire [the] worse it gets...I have said to you, Arlene and TJ [Timothy Johnston] that I will fit my story in with the party narrative and what is best for party”.

Mr Bell last week drew attention to that message, arguing that it showed Mr Cairns evidence should be distrusted.

When asked about the message, Mr Cairns said the reference the making his story “fit” was in “if they wanted to protect Mr Robinson, while I disagreed with that strategy, I was still willing to help”.