If the car stays at the garage to do the re-test, and the work is completed within 10 days the re-test is free. If you take the car away and then return it the garage will re-test it, but usually at a cost. Commonly half of the price you paid in the first place. A leaking CV joint gaiter putting grease onto the brakes will need a new CV gaiter, grease, a set of brake pads and a lot of spray brake cleaner. Changing a CV boot is a pretty straight forward process, but can be a pain in the **** job sometimes. Takes about an hour, maybe an hour and a half, plus time to change the brake pads on the other side.
I'd say £120 to get it done is fair. Make sure the garage use good brake pads, not cheap ones. The difference in performance is noticeable. Also check they intend to replace all 4 pads, some dodgy garages will just take out the ones you have and clean them off. That just ain;t good enough!