The problem with these sorts of jobs is they are too small really. The actual time taken to do it could be around an hour, but by the time he has got the part, travelled to your house, fitted it and been paid much more time is used up. My guess is that you find such a tradesman will want around �70. Why not ring around a few handymen - you don't really need plumbing skills to do this.
When mine broke not long ago I phoned a 'handyman' out of the local paper and he charged me �40 to replace all the innards of the toilet (I bought the parts myself from B&Q for about a tenner).
buildersmate is spot on around the �70 mark. As he says get a local handyman in, as we plumbers do not come cheap and have a call out charge no matter how simple or small the job is