RJS wrote:
The cause of a loose socket has been traced to loose bricks (or their remains). The socket pattress was held loosely in by plaster around its edges.

As the fragments of brick were doing nothing more than sitting on the cavity wall foam I have removed them.

I now have a rather intimindating void to fill.

I often get this problem in my house because the inner leaf is very poor
brickwork. I've found that it is perfectly satisfactory to clean out all
the loose debris and then use a filler/adhesive such as Gunnanail or
Nomorenails, setting the backbox in it. If the side knockouts are
knocked in a bit they provide a key. If the backbox won't sit in
position put a length of ply or something about an inch wide over the
hole, with the backbox fixed to it using the normal screw holes. The ply
can be tacked to the adjacent wall if needs be. I've found that sockets
fixed that way are absolutely firm. The filler should of course be below
the wall face to allow for a bit of polyfilla.

Bill