Arlene Foster said that no options about the cap were put in front of her by officials

Having done so, it is difficult to give credence to the official explanation for the absence of cost controls in the Northern Ireland scheme.

In February, a senior DETI official gave an Assembly committee the impression that putting in cost controls would have been a complex and time-consuming task.

When asked why such key measures – degression of payments as demand increased and a cap (set at 1,314 hours per year in GB) – were not included, he said: “At that point, the Northern Ireland scheme was under performing and we were not using up what you might say was free money in terms of AME to bring it in.

Sign up to our daily newsletter The i newsletter cut through the noise Sign up Thanks for signing up! Sorry, there seem to be some issues. Please try again later. Submitting...

“So the minister decided that the priority should be on the introduction of the domestic RHI scheme so resources were devoted to that. It was not a case that we said ‘well, we’re not going to do x, y and z...”

However, having read through the two pieces of legislation that defence now seems difficult to sustain.

The Northern Ireland scheme was overwhelmingly a copy and paste of its GB counterpart. The vast majority of what changes there were involved references to Northern Ireland, rather than GB, legislation and changing ‘authority’ to ‘department’.

And yet, when one gets to Part 5, section 37 of the GB regulations, the copy and paste stops. There are 107 missing words and it is those missing words which will, according to Stormont officials’ and Audit Office estimates, now cost Northern Ireland an additional £405m. That’s about £3.7 million per word.

My estimate is that about 98% of the Northern Ireland legislation is directly copied from the GB regulations. Scrolling down page after page there is either no change whatsoever, or minor tidying up such as Northern Ireland-specific references.

Therefore, someone appears to have taken a conscious decision to remove cost controls. It was not, as the official explanation goes, some complex piece of legislative drafting which couldn’t be cobbled together in the available time. It was, quite literally, a decision to copy and paste.

It is not remotely credible that even junior civil servants (and these were not junior civil servants), a minister who is a trained lawyer or her special advisor who has a Phd would fail to grasp the importance of a cap and accidentally leave it out.

Although Arlene Foster signed off on the regulations at a point where the cap had been removed, it is not yet clear whether the decision to remove it was instigated by her, by her advisor or by the officials acting on their own instincts.

The most benign explanation for what appears to have been a deliberate act was that there was a decision – and it surely would have to have been a political one – that as they seemingly believed (and clearly mistakenly) that the money was coming direct from Westminster rather than out of Stormont’s budget that this was an easy way to get more money into Northern Ireland.

Did the official’s testimony to the Assembly earlier this year when he referred to it as “what you might say was free money” allude to that?

If so, it still does not fully explain why the cap was removed. The tariff could have been set higher, or the scheme could have been aggressively marketed if uptake was low, without changing the rules in a way which incentivised people to leave boilers running 24 hours a day.

Mrs Foster has said that no options about the cap were put in front of her by officials.

Whoever was to blame, that catastrophic decision is at the core of this situation.