Further to this issue:
While cleaning the filters made things a lot better, and the jets aren't so near the step when running right, it was still a bit rich, and yet needed lots of choke for longer than seemed reasonable, even at like 12C ambient. Though, otherwise running ok when hot.
So, thinking the problem is still related to the air filters, I looked for the cheapest option for me to replace and found these:
https://www.ebay.co.uk/itm/AeroLine-Sta ... 2749.l2649
They look a bit shallow compared to the K&Ns, but they still offer more area than the originals, and the sponge filters can be washed or replaced reasonably cheap. But at £18, I though I'd just get a pair. I've yet to do a full reset of the carburation, but the need to run with the choke too far out for too long seems to have been solved. There's a bit much throttle lift off the choke now, but that's an easy fix (but why do I need a 7mm spanner to do it?).
I can't say there're entirely simple to fit; at least, not as simple a swap as I expected:
The edges of the expanded steel round the foams are razor sharp, so the latex gloves were shredded and I did get cut a bit.
The holes for the screws that hold the front and back together are like a millimeter too close together, so one head or the other binds on the edge of the carb's front face.
Also (presumably for cost) these slotted pan head screws are not captive, like on the K&Ns. That means that, to tighten the front nuts, you need a screwdriver no more than about 25mm long to get in between the plate and the carb - it's too tight for even one for a socket driver.
So I've ordered some M6 thin nuts to lock the screws to the back plate. That will mean there'll be nuts inside the filter enclosure, but they'd have to run down 30mm of screw to come loose and get in the engine. So I'm not over worried about that.
Graham