I have used them (and recommended them to friends) several times.
The guys they use are freelance fitters, not direct employees of GAP - hence their ability to cover the whole of the UK.
Everytime I call them I am very upfront and tell them "Send me the best person you have, not some monkey with a screw driver". I always end up with top class people who di a top classs job.
I would have refused payment, or rejected the intsall if the had damaged my car.