After a week of blow torching and hammering I've decided to take the frame into Carrick Engineering in Prestwick, for a modest amount they'll remove the broken bolt and sleeve using acetylene torches and brute force meaning I can then strip the frame and feel like I'm starting to something constructive rather than destructive.

Recognising I don't have the appropriate tools to do these jobs is something that I'll realise quicker in the future though half the fun is in trying yourself.
At least its not occupying all of my time now and I'll start with stripping the powdercoat first.
I'll fire up the blowtorch and hope it doesn't rain all weekend.The other alternative is traditional paint stripper which is fairly ineffective against powdercoat so I'm told or gasket stripper which is fierce but quite nasty stuff to work with.

The bearing cups are off and I removed the steering lock, annoyingly the little lock cover you can see in the previous pictures went flying into space when I hammered it off, I then wasted the best part of an hour looking for it before resigning myself to making another one from brass sheet.
I was all ready to drill out the lock and just quickly checked a website for info on the exact drilling point to avoid the hardened tumblers inside when I noticed 'the quick way'- number 3 on here-
http://w6rec.com/duane/bmw/lock/index.htm
14 seconds later the lock was out.

Just as one sheared bolt is fixed another two come along to try my patience, I think I'll grind off the captive nuts on this though and get new ones tacked on in the same position.

I have found that you can get clamp that encircles the pipe with a captive bolt attached and that may be an easier option meaning I just need to grind of the broken bolts and clean up any remaining weld.