Thanks for reply - its already got a 6 speed on it, so want to go up to 8 - so screw on is the way then...?
No - rear end of the bike is too narrow and the existing hub will be too wide. You would be better off buying a new set of wheels with a cassette freehub and go from there - you could go 9spd with a change of shifter and rear mech (most Shimano rear mechs will work with 6/7/8/9spd indexed shifters and matching cassette)
Depending on budget...
Have a good read of this site: http://www.sheldonbrown.com/
hours of reading there that will help with what you want to do.