You do have a defect, it is the ongoing cracking internally. Bellway cannot ignore this.
However, I would add that, on the proviso it is done proper workmanlike manner, fitting a new soffit over the old one, in one piece would make it look better.
But I doubt they would take the roof off to do it in one piece and I very much doubt Bellway has anyone skilled enough to cut and fit the new fascia in one piece accurately scribed to the window.
It will be done in several pieces with joiner strips and, in all probability, may end up looking even worse.
I suggest you write back and ask how they propose to fit the soffit in one piece (subject to 8' x 4' sheet sizes) and they supply you with a detailed method statement on how exactly the work will be carried out.
If nothing else it will waste their time!