Thanks for all the replies folks.
I'll keep this thread in favourites for future reference.
Ok, now I'm going to annoy you all. The hole was a mess, no threads until the last few. Tightening the bolt just ended up with it coming loose again. I only have a metric tap and die set so drilled the hole out to 9.5mm and tapped it to accept a piece of 10mm threaded bar to form a stud. Stud was secured by screwing it in using the two nut method and copious amounts of thread lock. Once assembled, an angle grinder took care of excess stud. Nut bolts up nice and tight. I had to drill out the clamp a little for it to accept the stud - didn't want to do that but it was that or the summer off the road. This was a temp replacement whilst I fix the original anyway, which leads me to my next question.
The mudguard studs are goosed. They appear to be welded to the lower fork leg. Obviously, I want to take care with any drilling, as I don't want to go into the tube!!! Thoughts are taking it to an engineer's workshop to get the studs removed and new studs welded in (don't have a lot of equipment, expertise, or welding stuff).
Thoughts always welcome!
By the way, when I have time, are there any helicoils that are metric on the outside and imperial on the inside to change the metric fix. To be honest, BSA seemed to use all sorts of nut and bolt sizes at different times, so I don't really know the actual size of any given fastener.