Thanks for the link and tip. That was the right cam, found part number 30992 on the internet in the end. Missed the auction unfortunately, and also I'm based in the UK, not sure if it was available via uk eBay. I glued the part with "bondloc steel epoxy", and left it in a clamp for a few days. I can't break it apart by hand now so hopefully that's it done now...we'll see! hate to think what damage it'd do to the breaker etc if it broke while the engine was running.
Also bought some of that JB Weld for next time, sounds like good stuff!
Thanks again,
Rich