lydia said..
Having pulled one out to sandblast and re-galvanise, the frame is put in before the bunk furniture so you have to cut the bunk fronts on one side to get it out.
True.
When I repaired mine I cut out the rusty bits and welded in 10mm stainless plate. It's highly unlikely the bits behind the bunks would be in bad shape but if they were I would make the whole lot in stainless in 3 parts and weld it together in situ.