I know there isn't, I was playing the part of the police officer that you speak to when saying there's more.
Sorry, didn't see your police badge.
If you hand them more money must they not take you at your word? They had obviously already taken the £60 found money from you. Ideally the person who lost the money would say he only lost £60 and not claim the £20 marked note. The police officers would then return the £20 note to you after 28 days.