When handing over a £20 note for beer, check and remember the last three ID digits - then when the bastards give you £5 change and you complain and they go and get the manager and fuck you over - you'll have evidence of the note on the top of their till trays.
that happened to me in the walkabout in swindon. fucking pissed me right off. they said come back at the end of the night and took my details etc, but it was pure bollocks. I ran out of cash 20 mins later and went home