Just to follow up on this, I got a 600mm length of seamless 25mm x 3mm wall tube. The cut column fit reasonable well inside the tube, so I welded it up.
The result, although not neatest of welds, seems to be strong - I turned the steering to full lock and put enough pressure on it to flex the steering wheel with no movement elsewhere!
There is also enough clearance on the upright.
I may add some 6mm dowel through the tube/cut column for additional peace of mine - What do you think?
Some pics:
