Interesting point here - my Mrs went into the local Costcutter just now. Some bloke was trying to pay with what turned out to be a forged £50 note.
The staff wouldn't let him have it back and he went into one and became very abusive. So what's the score with forged notes? I suppose technically it's his property and they should have returned it but surely the only reason he wanted it back was to try offloading it somewhere else?