Monday’s disclosure that Treasury and Resources got its sums wrong in the all-important capital prioritisation Billet d’Etat report that was to have been discussed next week is of more significance than might first have been thought.

The reason, quite apart from neatly filleting an alternative funding package that was growing in appeal to States members, is that the error – if that is what it was – calls into question T&R’s competence and credibility.

In plumping for an expensive ‘external’ borrowing option, the department claimed that it had looked at other possibilities and ruled them out.

Specifically, the minister stated that ‘the department has also considered internal borrowing as part of the process’. The accuracy of that now looks exceptionally questionable.

In seeking to justify its mistake, T&R says that the £100m. figure in the States ‘cash pool’ that would fund the internal borrowing was merely a piece of folklore handed down from 2003.

As an explanation, it is scarcely credible. For a start, the original figure was £105m. and even by T&R’s standards, writing off £5m. is either profligate or sloppy. More seriously, did no one consider that a six-year-old figure might need a little updating?

Where this starts to assume particular significance, however, is over the department’s assertion that it looked at other funding methods including the cash pool.

We now know that it cannot have done so.

If it had, the first thing it would need to establish was how much money was in the cash pool and how much might be syphoned off for the States to use. The ‘mistake’ in the Billet nails that myth: no checking was carried out so no proper consideration was given to internal funding.

What does that mean for the believability of its allied claim that other funding methods had been examined and ruled out?

If there is an explanation for all this, Treasury needs to set it out very clearly indeed. Until it does, the suspicion is that the board was fixated on commercial borrowing and much of its Billet cannot be taken at face value.

That is hugely damaging and it cannot afford any more mistakes.