Andrew McCormick, who was the most senior official working under Jonathan Bell when the RHI scheme ran out of control in 2015, gives a unique window into the inner workings of Northern Ireland’s most successful – and one of its most disciplined – parties.

In witness statements and documentation which run to more than 1,000 pages, Dr McCormick set out a spat over whether Mr Bell was invited to an event with Boris Johnson, a late night out for Mr Bell in New York which led to him being unable to adequately discharge his duties the following morning, Mr Bell’s recording of as yet unreleased audio tapes involving his then DUP Assembly colleague Carla Lockhart and the at times dysfunctional relationship between the minister and the DUP Spad who had been assigned to him.

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Former DUP MLA Jonathan Bell

It has also emerged that Mr Bell’s Spad, Timothy Cairns, had alleged that he was being bullied by the minister, and that Mr Bell had tried to break his finger and swung a punch at him, something of which Dr McCormick said he was unaware.

He said of that: “It would be incongruous to expect that such a delicate internal party matter would come to the attention of the permanent civil service.”

As the inquiry moves into what may be its final few months and examines the delays in reining in the RHI scheme, former minister Jonathan Bell’s credibility will be central to establishing whether some of his allegations against the party are accurate and much of the evidence released last night relates to how he operated.

Large tracts of Dr McCormick’s statements refer to Mr Bell’s idiosyncratic style as minister and to clashes during his tenure, several of which related to foreign trips. Dr McCormick – who is now heading up Stormont’s Brexit planning – said that “we in DETI and Invest NI reflected that Jonathan Bell’s trip to the USA in January 2016 leading an Invest NI trade mission had not gone well”.

He said that “in a number of subsequent conversations with Invest NI colleagues, they commented on Jonathan Bell’s limited contribution to the projection of Northern Ireland’s interests in relation to foreign direct investment and trade (indeed they had been aware of his limited capacity to contribute effectively, and the risk of him making inappropriate or unhelpful comments even before the trip took place).”

Dr McCormick went on to say that “it was noted that Jonathan Bell was visibly tired at a key meeting on Friday 28 January 2016. The context for this was that I had accompanied him and Niall Gibbons of Tourism Ireland for a meal and some drinks the previous evening, and although I left them when I felt the need to sleep, I was told later that Jonathan Bell had had quite a late night...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.”

Dr McCormick said that around that time it became clear that Arlene Foster “did not wish Minister Bell to play a role in those [US] campaigns as extensively as would be normal”.

He said that in February 2016, the following month, there had been plans for Mr Bell to lead another trade mission to the US over St Patrick’s Day, The St Patrick’s Day visit was dropped by Invest NI, with “indications that Jonathan Bell resented this decision” and, in what the civil servant took to be a direct result of that resentment, “he delayed providing routine authorisation of Invest NI briefing and travel in support of the First Minister and the deputy First Minister”.

Setting out how the intra-DUP dispute escalated, Dr McCormick said: “At one point, his refusal to approve travel by Invest NI personnel to accompany the First Minister and the deputy First Minister left Invest NI having to consider cancelling a visit to the USA, and I had to contemplate seeking a Ministerial Direction [a rarely-used nuclear option for permanent secretaries if they believe a minister’s actions do not represent value for money for taxpayers] as that cancellation would have been bad value for money”.

Even after being over-ruled by Mrs Foster, Mr Bell attempted to fly using his own British Airways frequent flyer points, but this did not prove possible.

Dr McCormick observed: “The inference that I took at the time from this exchange was that the First Minister had decided to stop Jonathan Bell from travelling – despite the established convention that successive DETI ministers have played a major and continuous role in representing Northern Ireland overseas.”