....right....got it!
It always amazes me how these things can happen. The required measurements are very simple (no rocket science involved). Perhaps the architect/designer got it wrong, or the builder's carpenter - it shouldn't matter to you anyway.
I assume NHBC did the Building Regs inspections in this case (not the local Building Control?)
Whatever - the builder got the measurements wrong, and NHBC passed it as correct. They'll put it right - or else!
Moving the trimmer might have ramifications in the final dimensions of the room or landing above, but, it should be fixable.
In my humble opinion - I would get it physically resolved before even thinking about getting the mortgage company involved. You'll only spook them, and get into another level of legal proceedure..............good luck :o)