On Wednesday, May 12, 2021 at 8:24:35 PM UTC+1, JohnP wrote:
Annoying situation. My house has locations for 2 wall lights with
architrave boxes for the wiring.

We bought 2 wall lights and annoyingly the the back plate (strip with
turned up ends) is shorter so I can only use one hole. With the stiffness
of the cable, it is difficult to get the wall light flat against the wall.

Have I missed a trick?

Extend the backplate/strip with a piece of similar thickness steel strip by pop riveting it onto to the back then drill a hole suitably spaced about 58mm between centres should do it and a bit of a sloppy hole 4mm+ should give you a bit of waggle room.

Richard