I have 5 downlighters in the bathroom that have 50w High voltage halogen
lamps in them (GU10).

I had a dimmer switch on them. I can't remember what it was rated at but it
was above 250w. The switch only lasted a few months and then burnt itself
out.
The switch was old so may just have been a coincidence that it burnt out but
it was pretty scary for my wife to find that at the top of the stairs there
was a smell of burning plastic but she couldn't see what it was. The smoke
had been going up the void in the wall!!!

Is there a specific dimmer switch I should be fitting?

Cheers
--
Steven
Don't give me any cheek if replying personally ;o)