It's out! Success!
Been soaking it with Plusgas for months and brought it down to the vehicle dismantlers with me.
Tried the vice but it still wouldn't shift, so the nut was welded to the bottom bracket, hit around the outside of the BB with a copper punch then the vice still wouldn't work, but a big set of stilsons and two people's effort finally budged it!
The next step would have been heat, but I'm glad to have avoided that.
A good degreasing, then off to Triple S.