In some respects it is hard to understand HMRC concluding that Rayner was not careless. Dan Neidle has very clearly set out why he considers she was careless. OTOH, as he admits, neither he nor we know the whole facts of the case. And as he also makes clear:
None of this is to suggest any impropriety on Ms Rayner's part. There is no evidence she tried to avoid or evade tax – this was (in our view, and on the facts as we know them) a careless mistake.
Also crucially, We don't expect HMRC would be influenced by Ms Rayner's position - they were, after all, able to independently investigate a sitting Chancellor of the Exchequer.
Ms Rayner therefore had to pay an additional £40,000 (and, we expect, about £3,000 of interest).
That should be an end to it but the right wing media will never let up until they have destroyed her career and some here won't be happy until that happens.