You need to ask for the money
1. It's (understandably) annoying you, and has the possibility of eating away at a good friendship
2. If it was you, and you'd genuinely forgotten, you'd hate to realise at a later date that you owed the money for so long (and that your pal had been to worried to ask you.
3 By not asking, you've ruined the memory of a good night out
3. if you're just straight up and say something along the lines of "oh that's just reminded me..... are you ok to give me that £30 you owe me?".... the conversation will move on to "oh sorry - here you are" without him even thinking "what had the previous conversation got to do with £30"
and erm....4 and 5......