Remove heads, remove valve body retaining nut, remove valve body from under bath, unscrew cartridge, re-assemble Easy providing some fool has not installed the bath against the wall. If they have the whole bath has to come out....
you'll be surprised at the amount of people who insist the taps go to the wall, dispite warnings of maintenance problems, i just say "ok its your call, but when they need attention, don't call me " and i put it in writing in the quote,
Cheers and yes some fool has put the taps against the wall, when i looked and tried everything i thought there's no way the bath has to come out to change a tap cartridge oohh dear
It is a popular option to put the taps against the wall until repairs need doing - then it becomes an expensive nightmare!