Tyre / tire size

Not having the same tyre size front and back is perfectly fine, you shouldn't have any problems. The only downside to having a 23c tyre if you previously had a 28c is that you will probably have a slightly less comfortable ride but thats about it.

Thanks mate - much appreciated. I wasn't sure if rim sizes differed somehow ... I will be having words with Cycle Surgery though to ask why they gave me different sizes!