[HPsauce]

Another bit of mental gymnastics that might help anyone else thinking of doing something similar, i.e. fitting the camera above the number plate.

The plastic backing plate, as noted above, is fixed with a few screws and the numberplate lights are fitted separately (and tightly) directly to the metal boot lid, presumably to keep them watertight. The plate itself is not sealed around the edge (though someone here has done that) so water can, and will, find it's way behind it.
Even if sealed all round the edge sprayed water can enter around the numberplate lights, there's a significant gap there.
There are small sealing discs around each fixing screw location because of that. I suspect degradation of those (mainly the larger two for the two big numberplate screws) is probably to blame for the water I get inside the boot lid, so I'll renew all those when I do this job properly.

So, I'm going to mount the camera on the plastic backing plate, ensuring it is as waterproof as possible - the cable entry isn't clever so I'll seal that. But if there's a hole for the cable I won't worry much, though I may put a grommet on it.
But beyond that I'll concentrate on providing a new waterproof entry (grommets, sealer etc.) for the cable through the metal behind it, making sure any connections are behind the seals I use.