Just re-wallpapered. Old wall lights were taken down and sold. Behind each was a deep "Architrave Switch Box" - The old lights has a back plate that had the same hole centres - must be standard I thought.
Ordered new lights - pack plate tiny - doesn't straddle the screw holes - or even the box in a vertical plane.
Turned through 90 degrees it straddles the box - but the fixing holes are right on the edge of the aperture in the plasterboard. Need to be careful not to mark the walls........................