This is the sort of thing I'm looking for. I'm certain you can get it as
there was one in a friends house when she moved in.

The problem is due to lack of space. The available space within the flue for
mounting is a brick's depth, a fairly generous width and about a foot in
height. I think a 100mm hole will also weaken the wall considerably, so
something that can take a smaller bore than the standard 100mm would be
good.

Can I ask why you blocked the flue off? You need to watch the chimeny
breast above doesn't get damp.

It wasn't me. The chimney from the kitchen rose through bedroom 3. This was
removed by previous owners as the extension of the upstairs bathroom caused
the chimney to jut out right next to where the door had been moved to. The
chimney stack is now chopped off in the loft space. The chimney in the
kitchen was untouched.

Since I moved in, we have opened out the hole to be big enough for a 90cm
range cooker. (10cm wider and 80cm taller than it was). Looking up what
remains of the flue, we can see the floorboards in bedroom 3 above.
Obviously, we need to block this off more effectively to prevent smells
rising.

The only stagnant air will be between the multiboard and the floorboards,
which are very gappy. I can't see it being much different to a standard
floor cavity, with plasterboard and little ventilation.

I think you definitely need a extractor. In short, steam and smells. Will
make for a much more pleasant environment. I don't think it would be a good
idea to have the steam constantly in the chimney breast with nowhere to go,
unless it is well sealed all round to keep damp out.

It'll be well sealed. The only thing is that I've never really used kitchen
extractors simply because every single one I've encountered has been a
"recirculating" type (read: completely f***ing useless).

Christian.