Hey Alex,
Wouldn't you just know it - the first time it drops to minus 4 and we are all botrhered about keeping the vans on the road.
Have you thought about coming up with a submersible pond heater-type gadget that we could all just stick on an extension lead and drop in the tank overnight. I for one am not keen on the drilling-a-hole-in-my-tank-immersion-heater approach. Something designed for window cleaners - rather than us all adapting a garden supplies company product. It would have to heat the water to a temperature that would not risk shocking the glass and thus risk cracking it, but would be warm enough for a large pane to be cleaned before the water freezes as it comes out of the brush, say 10c.
So come on mate, get cracking - i'd buy one and I bet a few on here would too...