Can anyone advise how i can fix a one off gap in the corner of my door so the gap is no more than 4mm all the way around? The door is fine and fits the frame ok but the top corner (latch side) has a 6mm gap where the rest is 4mm or under, how/what can I do to adjust it to only affect the top corner?

Fixing door to frame perimeter gaps can be tricky. In the case you describe the gap is 6mm at the top corner of the closing edge and the best way to resolve would be to adjust the door frame hanging jamb by removing architraves, loosening the frame and using timber packers/wedges to adjust the jamb from the back. But if the jamb joint is morticed or let-in to the head in some way this will be time consuming and the joint may have to be widened and done again. 

It may be possible to pack behind the top hinge with non-combustible material but the risk is that doing so would open up a larger gap and the hinge side.

Before you begin, check with your Fire Risk Assessment that the door is critical to fire safety and that this work needs to be done. 

Can the frame be cut in that corner with a multitool and wedge the frame over and refit new screws on an angle at the top side cotber