Dave Plowman (News) laid this down on his screen :
In article ,
Phil L wrote:
I've tried gluing the back box in but as soon as there is any movement
of the cables, which there obviously is when connecting the switch,
the glue can't hold and the box moves about again.

Remove the box, and fit a couple of wood battens either side of the
opening (1" x 1" will be more than adequate) to the back of the
plasterboard, their edges flush with the opening. Screw them in place
using screws through the plasterboard and make good the heads - non
rusting screws being best.

Then screw the box to the battens using small countersunk head screws that
won't foul on the fitting in any way. You may well have to drill holes in
the box sides for this.

This method provides an extremely strong fixing for any steel backbox
into plasterboard.

If the plasterboard were tiled or well decorated, you might get away with
glueing the battens to the back of it and clamping until the glue sets. If
you're lucky. ;-)

Excellent, cheers Dave