Thank you Stevie B and Gixer1460. I've done as suggested and reset the adjustment from 1/2 turn to 1/4, and it's made a tremendous difference. I might try reducing it a bit more yet; as long as there's still a definite bit of clearance I can't see that it can do any harm can it?
Also, I think that part of the problem is that the bike races a bit when I first start it, so today I let it warm up so that it'd tick over at about 1000rpm, maybe a bit less, before I put it in gear. I'm going to take the tank off to do the petrol tap soon, so I'll balance the carbs etc. while I'm at it.
Thanks for helping, the problem is much improved now, more or less as good as I'd expect.
Cheers, Brian.