There was a lot to chuck away once the kicker cover was off: as well as the broken starter gear, the mainshaft gear, starter clutch spring and clutch keys were all replaced. The bumper plate was horribly mangled and was also renewed.
I took the cover to Pete's workshop so he could help me press out the kicker shaft bushes, but there had been a previous repair done on the inside of the cover and Pete advised leaving it all alone in case pressing out the bushes cracked the weak spot.
Anyway, it's all back together and seems OK. I had a blast around the country lanes tonight and after coughing up a bit of phlegm (French fuel) the Pan ran great.
I have re-corked the timing hole using an improved rubber bung (my French wine cork sits nonchalently on the mantlepiece in the office), and will be visiting Pete's friend Rob (see Pete's posts on Rob's handbuilt Railton on Eat the Rich) to see if he can wave his magic wand over it and effect a permanent fix.
Now I just have to work out why the bike is still idling so high when the Linkert's idle screw is back to the stop...