Julian...thanks. Something you've said there, i.e. "wire goes tight when retarding" has made the penny drop! I'm certain I read it somewhere that tightening the cable advances the ignition and I've been working on that basis. However, from a search on this site, I now know that for a K2F mag, it's the other way round, although the opposite for some other types of magneto (another basic error I hear you say!). So my points 'set position' has been wrong anyway regardless of whether they moved or not when re-tightening.
Crikey, I'm definitely learning the hard way here!
Steve