Steve,

How do these work? Do they literally just smash into the wall
creating a socket sized rectangular hole? Is it a one step process or
do you need to pre-drill or do any extra chiselling out at the end?

It's a three (and possible a four) stage process:

Having marked out the position of the box, you drill a central hole which
will eventually take a wallplug for the securing screw.

You then use a two-inch circular face cutter (supplied in the full kit) to
cut a round hole to the depth needed by the box. The cutter has a central
guiding rod which fits into the pre-drilled hole.

With the SDS drill set to hammer only, you use the box cutter (which also
has a central guide rod) to cut the edges of the square (or rectangular)
hole to the correct depth. If you're lucky, the masonry between the edges
and the round hole will just break away. If you're unlucky you move on to
stage four, and...

Chisel away the excess brick.

I found that the system works well. It does, though, produce a fair old bit
of noise, vibration and dust.

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