HMRC lost original paperwork

A new client of mine had his self assessment affairs investigated. Subsequently he has been asked to provide documentary evidence. HMRC have lost the paperwork he sent them which, unfortunately, was the original documentary evidence. He is now being harassed to pay a large sum of money for which no calculations have been provided as the officer investigating has since left HMRC and has left no records or his work has also been lost.

I believe my client to have submitted accurate returns and am concerned that his is being unfairly charged. HMRC are being extremely stubborn believe it or not.

Does anybody know of a way to resolve this?

Thanks