Thanks for the reply.
1) So I need to remove the existing external vent? Won't this affect the cavity wall ventilation. It's a bungalow.
2) The fan will be in zone 3. Understood about the isolator, but can it be next to the fan itself with a pull cord. I think I have seen some that have a pull...
I want to fit a bathroom extractor fan.
There is an existing "passive" internal vent and corresponding external air brick.
Question: Can I pass the ventilation hose through the cavity and mount the external vent onto the air brick? I realise I will have to make a 100mm hole in the air brick...