Four hours later, (mainly spent collating all the paperwork from the four corners of the Universe) I complete my calculations. Bottom line - I owe them 42p. Which is what I expected to find

But what does the HMR&C calculation say? They're going to repay me £195.70 !!

So now I've got to (a) wait for their cheque which is "in the post" (b) bank it (c) wait for it to clear (d) write to them (I ain't phoning them at their rates) (e) wait three months (f) send them a reminder (g) wait another 3 months (h) get a fresh calculation from them and (i) repay them the money they shouldn't have given me in the first place.
I didn't ask for this. Why couldn't they just leave things alone? Ruddy barstewards!

(Yes I am going to bank the cheque. If I don't, they'll just get even more confused.)