The mapp gas heated the weld enough to soften it enough for it to let go of the mount but not fully melt it.
For some reason the left mount & plate both had bad arc welds under the bronze welds. Unfortunately when I got the left mount off there was a small patch of the tube attached to the mount & on the back of the tube was a crack. I'm thinking the best way to repair this would be to remove the damaged section of tube & turn up a replacement section that fits inside the original tube at each end

Hopefully I can unseize the swingarm pin from the frame in the next couple of days & take it over to Steve Roberts for him to have a look at sorting the bends & the damage to the tubes