Buying a prom suit in Cambridge??? under �100?

If I was you I would rent the suit. I've got a prom in a couple of days and thats what i'm going to do. You'll have plenty of change left over from �100 and you will have a much nicer high quality suit to wear. This way the assistant will be able to help choose a suit that looks good on you, and that fits properly. MOSS Bros do a good prom deal, and also have a few designs for narrower framed people, like many guys. Black is probably the best bet, it goes with anything and you can chose a tie to match the colour of your prom date!