What to do with counterfeit 20 pound notes?

Posted by MatchEffective903@reddit | AskUK | View on Reddit | 69 comments

I just sold a synthesiser on Facebook marketplace for £1000. The guy paid me in 20 pound notes. Whilst trying to pay them in at the post office I was told that £900 worth were fake. What am I supposed to do with them? Bin? Police? Self service checkout?