Well I've made some progress on this. I must say I find it amazing
that the hood is just too long for modern ceilings and will not even
adjust to the minimum stated in the specs. But I'm stuck with it so...

We decided that the bare minimum length might be ok. This would be
using just one of the stainless canopy sections (the inner one).
This would mean no cutting of the canopy, but would require extra
holes drilled into the canopy to fit the ceiling mount plate.

To see if this would work I made a model. A piece of cardboard the
same size as the glass part of the hood, with some polystyrene blocks
cut to fit into the canopy. I hung the single canopy on the ceiling
bracket and then attached the blocks to the cardboard (double sided
carpet tape) and pushed this into the canopy. This effectively made
something the same size and position as the final hood.
We decided this would be OK. With our low ceiling it was worst case
71cm over the worktop.

Adding the extra holes to the stainless steel canopy was easy. For
the record, the process I used was:
Put masking tape onto canopy.
Mark with pen where the holes are to be.
Support the canopy between two workbenches so that the benches go
inside the canopy. Use an offcut of wood under the end to be
drilled.
Use a new sharp self tapping screw and a hammer to make a punch hole
(or a centre punch of course)
Use a brand new HSS 2mm drill bit in a variable speed drill and go
slowly.
Once the first hold is through I then used a 3.5mm bit, and then a
6mm bit to finish.

So now I have the hood with 4 angle irons sticking up and the canopy
over the top with new holes aligned to pass through the canopy,
through the angle irons and into the ceiling hanger. You obviously
can't get to the inside of the canopy when mounted, so I tapped the
holes in the ceiling hanger to be M5 threads.

Finally, to make it easier to get the canopy/angles over the edges of
the ceiling plate I have packed the internal gap between the canopy
and angle irons with offcuts of MDF.

The last problem to overcome is how to lift the 30Kg hood up to the
ceiling while aligning it, and putting in 8x M5 bolts. I think 2 big
mates is the answer.

Painters10