Deric
There does appear to be a problem with the roof fitting on the Maxitrak Class 66.
A quick look around the web reveals more poorly fitted rooves than well fitted ones.
I think the solution will be for you to hack the GRP moulding until it looks ok in the hole.
Then apply a generous coat of body filler on the inside and hope it doesn't fall out.
Something of a bodge I think and not what you really expect for nearly £3k
But I don't think you'll get much help from Maxitrak, from my experience with them