Keith H may still be right and it may be a dumb question, but have you tried moving a complete gear tooth. You should find the correct firing point within the range of the lever, then decide if you need to adjust.
Thanks Peter I'll try that. Ian, that's the sort of thing I was thinking of but unfortunately I have no B series stuff and Richard, thanks for the offer but I'm in Australia.
I know that! I guess I should have mentioned that I am too as the email address tends to give the wrong impression, we met at last year's ABR, and I'm on Southbank.