Thanks guys, very much appreciate all your great suggestions, time is on my side and if the exhaust studs hold think I'll be removing swingarm to facilitate bench work. I'll keep you posted
Update: Success! Well that was a jolly couple of hours. Having failed with all the penertrating fluids, resorted to basics, drill off the head and drift out the remnants. Initially the drill bit was just skating off the bolt head, suspect potential work hardening from impact gun etc. Bolt head eventually gave in to firm attention from die grinder and then drill bit although when the bolt head was completely removed it was still not possible to drift out the remnants, using a tool punch and 4lb club hammer, it was decided to continue with the drill and then try drifting again as further drilling progress was made through the bolt. Eventually the bolt surrendered, drill bit appears to have gone off centre very slightly but the swingarm appears unharmed. Image shows remnant of bolt head uppermost, then washer beneath and finally nut / bolt and plate. cheers all.
^ unfortunately no, all the smaller bit sizes were just not long enough to reach through the swingarm, so it was 9mm all the way.
