I was keen not to screw the number plate to the bumper, mainly because I don't want unsightly screws showing on the plate, and drilling the plate is very "permanent". So I made a plate that is mounted to the bumper as per the normal method, and then the number plate is mounted on that plate.
So, metal plate measured, cut and mounted. It's smaller than a number plate so cannot be seen:

This was then painted up to keep it clean and tidy:
[url=https://flic.kr/p/CpzmsG]

And then the number plate fitted:

Much cleaner and tidier than screwing it on IMO.