Sadly... I think I've got to concede it's proper shagged...
Despite buying a bolt-on-able BB tool, soaking the BB it in WD40 for a couple of days, heating it pretty hot with my camping stove(?!) and attaching a 1.0m arm to my spanner, nowt doing.
Then I found another rust hole in one of the rear struts (below) so I'm thinking the corrosion zone is probably 10cm+ in all directions away from the BB...
think I'll break it and sell what bits I can salvage...