Agree this should be taken with a pinch of salt.
Part of what underlies this is that it has been nearly 3 years since the last spending review, and hence since departmental spending totals were set. A lot has happened since then, meaning that the allocations that looked generous in October 2021 now look much less so.
Just under half the black hole is because Labour has allocated an additional £9 billion to public sector pay. There would have been a hole there regardless of who was in power as the 2% budgeted for public sector pay rises was clearly inadequate, but it isn't clear that Labour needed to go as far as they have.
Another £6.4 billion is the cost of supporting asylum seekers. This is not included in the Home Office budget, where it should have been. Instead, the Tories funded it from the Treasury reserve, which is intended to cover unexpected and urgent demands on the public purse. Moving that into the Home Office budget whilst keeping the reserve at the same size means it adds to the black hole.
A further part of the black hole was due to the Tories announcing a number of new initiatives that were not included in departmental budgets. It seems all of these were going to be funded from the reserve.
The last government weren't cooking the books and some of the alleged black hole is entirely of Labour's own making, but the Tories did leave the new government a worse mess than they were expecting.
Both parties are at fault. The Tories who left a lot for Labour to clear up and weren't honest about the challenges faced, and Labour who knew broadly what these challenges were but kept quiet about them, saying nothing in their manifesto or in any of their pre-election statements. However, I can see no evidence that gets anywhere near proving that anyone committed a criminal offence.