Doubt if this update will be seen by many (any ?) but just in case someone Googles here and is interested wrt their own issues :
Finally got to call the builder. He came around, looked at it, and said it isn't roof spread, it's a good roof. Said that he can fix whatever is wrong but needs a structural engineer to say what the cause is.
I found a local one from
https://www.istructe.org/finding-a-structural-engineer/find-an-engineer and they have just visited.
She tells me that the footings are fine. The block walls rest on a brick layer near ground level and the cracks don't extend into them. All walls seem vertical and flat. No cracks near openings such as the door. Bathroom tiles & grouting look unaffected. Have it all a good look.
Says she believes the cracks are thermal cracks, and they can be filled when next decorating. The outside is probably best left to the builder to open & fill.
As a bonus I asked about the drive dropping near the wall. Says it is unconnected to the cracks, noted it is near the soil water pipe, in a dip (obviously) and that the water is likely to accumulate there and wash away bits. Should just need taking up, and redoing properly.
So that's a weight off my mind. I await the report (and the bill).