Oh jesus christ no it isnt a light dimmer switch, i know that's pretty damn dangerous - read all that on overgrow. It's a fan speed controller purchased from the hydro shop growell.co.uk. It's a potentiometer which although is not ideal, for low wattage 240v fans it is adequate. But as you say, some fans can't have the speed regulated in which case you need to use a variable frequency drive to regulate the frequency (surprisingly lol) - can get em off ebay or electronics shops. I decided to take the easier, debatebly less safe option but have had it running for a year now with no problems.
Good discussion man, think i'm gonna be at home on this board.

Oh and the module cost me about 12 pounds, I can link you if you can't find it.