That does surprise me Steve, London to Glasgow for Solar work, especially as my experience with this type of work is not great. I mentioned on another thread I noticed a big London firm round my way doing over head lines and I thought even that was strange but I guess a contract like that would probably dictate you would not be allowed to sub out but there is usually ways round anything. That's why I never cut my price when threatened with cheaper prices from outside companies, service. The few that I have lost have always resulted in a phone call requesting I send a squad to snag the job, they usually get very upset when I refuse.
That been said, I have never heard of the particular firm but they should maybe have a rep there to sign the job off as you complete it to try and stop that happening in the future. Not easy I know, as we had similar problems with a National window supplier and eventually fell out with them.