The gap is all to do with a bodge when the original doors were put in - they were not on the plinth of the house but on a separate bit of wood. Bodged at the top as well - there's an RSJ right along the top edge of the doors. This also meant that the doors were bridging - which goes some way to explaining the arctic conditions in the living room.
This meant that when the new doors were placed on the plinth, they fitted snugly withe the RSJ bit of wall but were lower than the outside edge - hence the gap.
The glazing guys are coming back to cover the gap with some trim (there was a communication issue about these) and the building control colleague has ressured us that we are unlikely to get any issues with thermal efficiency. And they will look fine!
Hoping that there'll be a good solution for recycling uPVC developed by the time the windows reach the end of their life...