If only these things had been spotted eight years ago, x79 :o(
As BM has said, surely NHBC, Zurich, or an Architect's certificate would have been in place, if only to arrange the mortgage? This should all have been picked up then. Sadly, I guess it's too late now without protracted legal battles.
Whether you go down this route, or even for your own peace of mind, I would encourage you to hire a Structural Engineer, or a Surveyor to look at it. Quite often, dramatic problems with roof structures need only simple solutions. It might cost you a couple of hundred pounds for his time, but I promise you, you'll feel a lot better for it.