Got the hole cut in the ceiling, and modified the vent to fit (it wasn't quite right, but it is now). I must say, when it's charged, I'm quite impressed with the little saw. I cleared up all the wreckage and plugged the fan in - no electrical fire. I turned it on, and it's not as noisy as I thought it was going to be, so I don't think we'd use the slower speed. Rather than get a High - Low - Off switch, I think I'll get a timer, so we can turn it on and walk away.
The bad news is that Tilly's frightened of the noise and the wind rushing past her. She's not listening to me saying she's unlikely to get sucked into the vent. Still, I'm sure she'll get used to it when we start using it more often :o)