Similar sort of thing here. I have just replaced the shower extractor fan / light, the original having a motor that wouldn't run any more due to collapsed bearings. The reason for this is that in its 'standard' form, the fan is controlled from the lighting switched live. That means that the fan always runs if the lights are on. The shower room is at the top of the stairs, and its lights cast a gentle light in the area through the fanlight over the door, so tend to get left on all evening until I finally go to bed (usually very late !!) and turn them off. So I've done a mod to the new one, and designed and built a little controller that senses when the shower pump motor is drawing current, and produces a DC voltage to drive a mains-rated relay that I managed to fit inside the fan run-on controller's casing. This means that the light on the fixture can just come on normally from the switched live, along with the other lights in there, and the fan won't run. When the shower is used, the fan starts, and runs as long as my sensor circuit detects that the pump is running. When it stops, the relay drops, and the run-on timer takes over to continue extraction for a few minutes. An added advantage of this is that in the summer when people take a shower without putting the lights on, the extraction will still come on.
I want the momentary switch to mount outside the shower room to bridge the relay contacts. That way, if I want extraction without the shower running (there's a bog in there as well, so use your imagination ... ) I can just stab the switch, which will start the extractor fan. It will then run for the run-on time controlled by its own timer. It does all work exactly as planned, and I have the wire running down the inside of the airing cupboard which is next to the shower room, so all that's now needed is to bring it through the wall, and obtain and connect the switch :-)
Arfa