Update: I managed to buy a really good oil pump from eBay, which arrived today, so after a good clean I fitted it. It actually has a ball valve, unlike the Mototrans one, which also has a weird housing on the outside of the pump body, possibly this is a kind of pressure relief valve? The body looks quite different to a Bologna one.
I had also discovered a lot of free play in the bevel coupling that I never noticed when fitting the head, so off with its head again and fitted a 0.01 undersized bush, which is pretty tight on the shafts and has taken out almost ( but not quite all) of the play, and while the head was off I helicoiled one of the cam end cap threads that was on its last legs.
Because the remnants of the exhaust port threads only have a tenuous grip, I thought it prudent to fashion a bracket to support the exhaust pipe, which is fixed next to the footrest.
The bike started first kick, which was nice, and I had left off the bevel cover to check that the oil pump was working; for a low-pressure pump, it sure pumps the oil at a furious rate! So that is it for now, until DVLA return to work and send me a log book, there is not much more to be done, except wheel the bike outside and give it a good wash off.