Thank you John, so simply put if I want to use my race wheels and the bike I've bought is currently a 10 speed bike I need to buy a 10 speed cassette for my race wheels to fit - if I've understood correctly. That makes sense thank you. I then just have to choose which one, 11/23, 11/25, 11/28, 12/23, 12/25 or 12/30... that's where I get a bit confused. The terrain is pretty flat here, only one short sharp hill (12% climb over 1/2 a mile) and another more gradual one, other than that it's just the wind I'm fighting! So I just need to figure out what cassette I need to pick....