through personal experience I would get tyres changed at National but nothing else.
Had a battery changed on tuesday (ordered on line with 10% discount) and was concerned at the fitters level of competence.
1. Saw him using a screw driver 2 sizes to big to remove the plastic press studs supporting part of the induction system which needed removal to get at the battery (2 minute job). I showed him how to remove the studs correctly at which point he asked me to sit in the waiting room!
2. Managed to find my way back out into the workshop

and noticed the battery was much larger than the factory item! when queried was advised that is what i ordered. The fitter removed the factory battery tray and manged to squeeze in the new battery!
3. 2 x battery clamps were broken during the work. They were rusty but i mentioned that wd40 might have helped I told the fitter to connect it up and i would sort it out myself. Heres the good bit.... the fitter connected up the negative lead first

then kept plonking around with the positive clamp on the battery terminal

... we all know the rule disconnect earth lead first reconnect last!
Anyway all sorted out now (battery clamps cost £6) and my car survived.
I suppose it's all down to the branch you use and the individual technician
As a positive I would like to say that i had a tyre replaced by National (10% internet discount)and the work was completed professionally.