I live in a Thirties mansion block and recently wanted to replace all the light switches with dimmers. When I removed one of the switches, the recess behind it was too shallow to contain the modern dimmer. Are there shallower switches or will I have to get all the recesses made deeper?

Most standard fittings are relatively thick unless you go for more complex electronic sensor switches, which we wouldn't recommend in your case. Any electrical work needs to be undertaken and certified by a qualified person (this is all to do with the Part P building regulations - find an electrician at www.niceic.org.uk). And also get the electrician to sort the recesses, too, to be on the safe side.