I've decided to have another go along the lines of how 'Fizzy' has suggested.
I've made a new gun metal cylinder. I have drilled the centre out some of the way to reduce the mass a bit to help with silver soldering. I'll solder the top and bottom plates on. Then put it in the lathe and drill and ream the cylinder to size, then solder to the valve body. I wouldn't think the final solder would distort the cylinder because the end away from the valve body will be a lot cooler and so hopefully keep it all round. As it was also up to temp for the initial soldering if it was going to distort then I'd like to think it would have happened then.
So that's my game plan.
Edited By Greg H on 12/12/2016 11:55:14