Why not just post the details on here so they are there for everybody to see should the same thing happen to someone else.

Hi All,
Not being coy - just didn't think there would be much interest in the details, that's all.
Anyway, the Raymarine ST4000 autopilot (and some other models, I guess) use a plastic planetary gearbox containing 9 identical 24 tooth 48 pitch gears, of which in my case, 6 were badly stripped when I went into Auto mode when the wheel lock was on.
After a lot of research, I found that Raymarine don't supply the gears or the gearbox, and the only source of a new gearbox that I turned up was approx $500 from a UK supplier.
Capn dan on cruisersforum.com generously gave me this link kimbroughracingproducts.com/store/index.php?dispatch=products.view&product_id=29872 which is a pinion gear for a model racing car - $6.50 US.
You need to cut off the end of the pinion gear to make it fit - otherwise it's a perfect replacement. Worked well for me.
regards,
Allan