I sold and posted an item recorded delivery but the bloke gave the wrong address

so it was returned to me but was lost coming back to me. On royalmail.co.uk it just says "being returned to sender" and has for about 3 weeks!
Is it my responsibility to chase it up? or the buyers? i have given him the tracking number but hes basically a dumbass
Whos to blame?
Cheers all!