Owain,

If you have a bank of sockets etc to mount, join the back boxes together
rigidly with conduit couplers and bushes etc into a single rigid assembly.
(Check distance between boxes to allow room for faceplate overlap). Work
against a raised lip on a workbench and you will have the top of the boxes
in a straight line as well as the backs on a plane surface. Then cement
the
whole lot into the all in one go.

That is an excellent suggestion. Thank you.

Bert
http://www.bertcoules.co.uk