The recommended Norton technique actually works quite well (long threaded rod) but I had to move the whole lot much further out as the rod had to go up the middle of the bearer rod, which was thoughtfully provided with a through hole at Bracebridge Street. It meant there was only 1/3 or so of the shroud engaging with the frame. I did consider the bedsheet......
Anyway, I've now successfully got the very rusty remains of the rod out of the slider so can start planning the rebuild. Sadly the bearer rod is a bigger diameter than I can get in my Chinese lathe so will need to find a helpful person to make that for me.
I'm so relieved to have managed to get it all apart however.