g at one for ~£30 from Screwfix.

2) I have an airbrick - well it's actually cast iron. I was planning to
knock that through and use that to install the fan. The question is, do
I need to install a duct through the wall and a vent on the exterior
wall? Or can I just install the fan on the interior wall only? I would
prefer not to touch the outside of the house as the cast iron "brick"
is quite ornate (and 80+ years old) and fits in with the rest of the
house.

There should be a air-pressure operated flap installed on the outside to
prevent wind blowing in when the fan is off, otherwise the bathroom will
be permanently cold. Doesn't the air brick vent the void between the
floors.

john2