Have a look at MK Grid stuff.

You can fit a 250W dimmer and a rocker switch into a single box.
However, it does need to be deeper than the very shallowest boxes
normally used for light switches


I had a look at the MK Grid stuff on TLC on your advice. Looked just the
ticket - till I checked the dimmer modules. £26+ VAT for a dimmer
module?!!!!

It would take quite few kilowatts of savings to justify that.

Now where's that bolster? :-)

Cheers

Sam