Service Stop are not a franchise, they are a middle man, all they do is take the booking and then they book it in to a local garage, you pay them, they pay the garage, we use to do work for Service Stop but stopped as they took all day to get authorisation on extras and then blamed us for not returning the vehicle back on time plus they tried to drive costs down so much it wasn't worth us doing!
Save yourself the hassle and just book it into a good local garage. If you are unsure on a decent garage go onto Unipart Car Care Center website and have a check on there, all UCCC garages are part of the Motor Codes and are all RAC inspected (I am a UCCC and we get marked on a lot of stuff to retain the franchise)
Also, Halfords garages are turning out to be pretty decent, the mechanics have access to good equipment and training and the company has a good pricing structure, I would still prefer a decent independent garage as generally they try harder to get and keep a good reputation compared to a large chain garage