"Hamish" wrote
| I fear that enlarging the rectangular hole may cause it to 'merge'
| with the bolt holes.

I surmise that the reason you have to enlarge the rectangular hole is
because you are using a sprung push-inwards exterior letter plate. If you
use a lift-flap exterior letter plate you probably won't have to do anything
about the hole inside.

Screwfix have quite a wide selection of letter plates under Ironmongery -
Door Furniture, including some lift-flap designs.

Owain