Just to close this one off, the suggestion of cutting down M5 threaded rod to size was a good one.
The only problem was that (on mine anyway) where the existing studs go into the frame, they are threadless/smooth, whereas I thought I could simply screw in the new piece of rod, there is no corresponding thread in the frame. They must have been brazed in somehow originally.
Anyway, it was a snug fit, and a little bit of metal epoxy putty did the trick and has held the new stud firmly in place.
Thanks for everyone's help on this one.
David