On Sun, 30 Jun 2019 09:41:30 +0100, Jeff Layman
wrote:

The OP needs to watch out if the cavity wall insulation is old
polystyrene beads which no longer stick to each other. When the hole
saw is removed the stuff will just pour out (especially if it's a
standard two-storey house and the hole is at the top of the ground
floor). I would advise him to have a vacuum cleaner ready and to shove
the ducting in a soon as possible after the hole saw is pulled out.

No insulation in the cavity but I didn't know that. I'll remember that
if I ever move house and have to do it again.