Thank you so much Tim, Richard and Giles. I do have (plenty) of aluminium, and more than enough steel sheet ideal for such a job, but for some reason I cowardly chickened out of attempting to roll either type into an accurately tapered tube!
In retrospect, it might have been a whole heap easier to try either, but had completely convinced myself that sticking ply together with offset joints to make what is in effect a continuous tube was preferable!
It was a monster to do, but I will say that the finished chimney is incredibly strong, and indeed may well survive some quite severe impacts in future? As to longevity, both ally and steel can corrode, the former sometimes very aggressively, while wood on the other hand has the potential to remain good for centuries - but with ply, it is usually the glue in between the veneers that fails!!
Some items of 16mm scale rolling stock in my small collection are quietly dismantling themselves after only (!) twenty - odd years!
Pete.