Orright, my update, a few points to share
Eventually got the flywheel off to find... drumroll, please... the needle roller bearing supporting the starter gear wheel was absent
Not broken, disintegrated or anything - completely missing. I've had the bike from new, so it's been missing ex-factory.
The seal saga; I too went in to my local SKF-badged dealership wanting a 19993 per this thread, which I cross-checked on the SKF web site. They sold me a NAK seal - saying the SKF part number didn't exist - cheaper and the comment was "they're all the same" but discovered later it had a thicker garter spring. Checked the web, yep the 19993 is listed. On the second try, I insisted the part number was valid - yep, they found it, there was one in the country and I would have it overnight... grrr! The SKF was clearly better made and more than double the price of the NAK
Starter clutch "pawls" were beginning to break down. They had flats top and bottom and some had lost their hardening. Also, the top surface either side of the U-channel for the spring had burred over, making the U-section look more like a teardrop with a narrow top. Removing the spring meant damaging it, I suspect that the other kinks in the spring were due to the burrs damaging it in use, like I did trying to extract the spring. Crudely filed out the channel so it wouldn't catch the new spring.
Everything else looked polished smooth, so I reassembled it with a new needle bearing.
Hit the starter, and... nothing!
Well okay, the starter motor was free-wheeling. Looks like the only reason I was getting the starter clutch to engage was because the gear wheel was flopping around on the crankshaft, I suppose the skewiff loading helped it to engage?
Anyway, now I am up for a new starter clutch, probably new starter gear wheel as the hub outer surface that engages the starter clutch looks very worn. I presume they are supposed to be flat, not having a trench in the hub, reminiscent of a worn brake disc? Hopefully not a new flywheel hub, too.
The bike sounds a little less rattly now, particularly at idle and when being kicked over. Or I could be imagining it.