For those who are interested, I have so far installed the cooker hood
without the chimney piece! The hool aligns perfectly with the top of
the adjacent kitchen units (and there will be a bridging piece of
melamined board with a cornicing). I have about 25cm between this and
the ceiling. I'm debating whether to bother cutting a 25cm piece of
chimney just for the sake of appearances, or whether to leave the hood
chimney-less (it looks pretty good as-is)..... most of the chimney
would be hidden behind the cornice anyway...

It will probably all depend on how good a job I can make of cutting the
chimney... I'll update when I've tried.