And here's where i am today.
For the tube i've used 40mm B&Q plumbing piping. Only costs 2 quid but the downside is gluing stuff to it. So i've copied Martin Bower again and wrapped strips of Plasticard round it.
Had a bit of a shock with the cones - they're FAR too flimsy and ther's no way i'm confident enough to cut one down, so am going with just the one. You can see here i've glued in a round plate for the engines to be built onto.