A client submitted a paper Self Assessment return which was processed incorrectly by HMRC, generating a refund. He phoned them up to confirm that this was correct and was assured that it was. He then used the funds to clear his mortgage.
Now HMRC are demanding it is paid back with backdated interest. I've looked at the HMRC guidance for this situation and it seems to fit the 3 criteria but can anyone quote any Case Law for this situation?