Thanks. Hopefully Barmaid will see it and can advise on Estoppel (although I recall that the claimant had to show he made a permanent change in lifestyle rather than a quick one-off purchase).
I recall twice I was paid twice (once as time sheets had been duplicated and then because two employers were confused as to who was paying me).I queried each and was told it was fine but I knew the admin person was wrong. However I knew it couldn't possibly be right so i just left the money in the bank and waited, and sure enough, over the following pay periods an adjustment was made and the money clawed back.
The devil may be in the detail here. It might be easier if he could tell us his outstanding balance 4,3,2 and 1 year ago so it is clearer whether it should have been obvious that an error had been made.
Citizens Advice may help here too if Barmaid can't