Make sure that the fan and motor box will fit inside the chimney, make
a custom filter box and shove the whole thing up inside.

I have a similar set up, with range in the chimney. The fireplace has been
widened and heightened. However, the flue stops abrubtly with some
floorboards in the room above.

I was planning just to block off the flue 30cm above the lintel with some
multiboard. My partner is vegetarian, so there won't be much meat cooking
going on.

Do you think I'll need an extractor as well? This would require about 3m
of
horizontal ducting above kitchen units and a centrifugal fan. Is it
possible
to get concealed centrifugal fans complete with grease filters? I can find
normal centrifugals easily.

I would always put in an extractor in a modern house, or an old house that
has been modernised. This is because there is not the level of airflow in
that you would find in an old house becase of all the drafts, from windows,
doors, gapped floorboards etc.. not to mention the forced draft caused by
the chimney. Because of all this the smells (aromas if you like!) from
cooking will hang around a lot longer - I think this is enough reason to
install a cooker hood.

From a building maintenence point of view you don't really want all the
moisture that is generated by cooking hanging around in your house either.

Even if you are not cooking meat you will still be using vegetable oils, I
expect, and this is IMHO the main source of the muck that is collected by a
cooker hood.

A hood also takes away some of the excess heat that is generated from
cooking. In general this can make the kitchen too hot or make the AC work
harder than it needs to; so another good reason to install a hood.

The only negatives that I can see are the extra electricity used for the
light and fan, and the draught and cooling effects, minimal.

I'm no expert, these are just my opinions, but I hope that I have been some
help.

Chris