I've finally bitten the bullet, and started sawin' (as the saying goes). I've managed to separate the boiler. I had to cut all round, and then pull, twist, etc to get the metal block out of the firebox. I don't know what the metal is, but I couldn't cut it with my razor saw.

I've just blu-tacked the boiler in place to take this photo. As you can see... well, it's a shade too long. About 1.5mm too long. I'm going to cut it just after the first boiler band, as that should get the front of the boiler/back of the smokebox in line with the back of the cylinders. I'm going to need some sort of extra weight in the boiler, as, compared to the Ixion one, it's a bit of a lightweight. Also I need to finish off the bottom of the boiler as well...

Hopefully, it's going to work out.