I eventually got down to the bit in the bike - removed petrol tank, disconnected battery, removed lower side panel, removed alternator (eventually! had to go and buy a long allen key to get to the bolts required) and then removed the old starter motor. This at least avoided the guidance in the service manual which says Remove Fuel Tank. Remove Carbs. Remove Engine Oil. Remove Engine.
So, out comes the old starter motor and I go to unbox the shiny new one. And, guess what? It is completely the wrong one. The picture in the advert was the correct one, much like the one closer to the camera below. Unfortunately the one further away in the picture is the one they sent. It has the wrong size, wrong fittings, wrong oil seal size and the wrong pinion. *sigh*.
So I may end up trying to repair old one instead. I have cleaned it all out (it was a big mess inside), checked the armature which seems to be connected in all the right places, and insulated in all of the (different) right places, and I have a kit of bits coming from the very capable Wemoto to replace the brushes and springy things, and I hope for a good result.
Meanwhile, catching the bus and stealing the car from wifey, who is very understanding.