Look up "Extra Statutory Concession A19 (ESC A19)" on the HMRC web site. It's to do with incorrect taxing by HMRC when they have been in posession of the relevant information.
Also, it could be worth research on the forums section of the "MoneySavingExpert.com" web site with regard to ESC A19. I wrote two letters earlier this year in response to an HMRC demand for over £2,500 and it was overturned using this regulation.
Good luck.