Well, I've got it fitted now, and it's not easy! Definitely comes with a bad language warning.
The original tube is 3/8 inch imperial, which is about 9.6mm. Imperial size tube is like chicken lips at the moment! The new tube is 10mm nominal diameter.
However despite the fact that it's nice and shiny, precision in size and round it aint. It certainly won't go through a 10mm hole, you need 10.2mm to get a sliding fit. Don't panic, this isn't more peerless mermaids and unicorns, it's a M12 tapping drill size.
The bit that needs opening is the clamp fitting where the tube exits the front of the steering box. The rest of the tube fits inside the steering column just fine.
Obviously you can't drill this out in situ. However, when you take the plate off, don't move the car or steering at all until you've bolted it back on. If the steering column moves, the bearing seats will get displaced, and are the very devil to re-seat, uphill, and you can't see the top one. It's possible on a Warwick, because you can get to it, but if it was a Peerless with the body on, you'll need to remove the steering box to sort it out.
The gland nut and brass olive need easing out as well, with the olive being the most difficult because it's tiny. I have a tapered broach I use, but the olive is split, not solid like a wedding ring. It needs to be fitted, or the tube will rotate, which was the point of the whole exercise.
All you need to do now is use a cutting risk to make a 60mm long slot in the top end and squeeze it down in a vice to fit into the keyway in the switch tube.
This would be an amusing one hour project if the steering box was on a bench. If it's in the car, and the body's on, think again.