Drilled the capscrew heads off, all degreased & checked.
Need to remove the mudguard bolts now, as I had to grind the heads off, as they were rotted - that will be a job for my engineer mate, as I've had a go and they are pretty stubborn so it will be a job for Harvey and his workshop. He managed to drill out and remove the grabrail bolt from the frame!
M8 sounds about right, but to be sure measure the thread length and pitch that come out of the damper tube ?, also don't forget new copper washers when you put new bolts in.