Yeah I've tried them they have some ridiculous system where you have to pay the 300 odd quid for some sort of scheme before they will come out. I'd rather find someone independent if possible as they were spectacularky grabby, rude and unhelpful.
You could phone round a few local independents. If you are in an area where a local builder put up lots of houses with the same system, the older ones, at least, will have experience. A local recommendation is best.