21t chainring seems a little odd, it's most likely 20 or 22 to be honest. Fairly normal gearing options are 22:18/22:19, or 20:17 (give or take a tooth or so, as obviously there's an element of personal preference to it) I see you're from Bristol. If possible, come out for a ride tomorrow, as a few of us should be out and about, and we'll be happy to explain a few things/get you on your way Oh, and what's the bike?!