Some success today. After reading through old threads I found this:
https://forum.triumphdolomite.co.uk/vie ... =4&t=11236
I started by grinding through the pins on the clutch fork using a 3" grinder.
Next i drilled the clutch arm from the other side in order to get a punch in to remove the broken bolt. Unfortunately this is where things got difficult, the bolt was so seized in it would not shift. I decided that brute force was my only option. Chocking a piece of wood behind the cross shaft arm to bell housing and using the pole from my trolley jack slide over one of the clutch fork arms I was able to apply enough force to break the remaining broken bolt. With the cross shaft free of the clutch fork I was able to persuade it out of the gearbox, with the gearbox still in the car.
Once on the bench, the pin was easier to remove after some gentle heating and a swift blow with the punch.
All in all not too bad a result. Release bearing carrier, and cross shaft totally un scathed, and a clutch fork that would be reusable with some new pins (which I haven’t been able to find - they are splined). I have ordered a replacement clutch fork from Chris Witor which I should have tomorrow or Friday, so the project is not derailed. The engine has to be in by the end of August (not running mind!).