Checked runout of mag and cam shafts and they run true. Checked the base and tips of the teeth for concentrically whilst fitted to the shafts, all good. However i did find some burring on the side of the mag sprocket teeth so clearly some wear evident. Have ordered two NOS sprockets from club spares as there were a reasonable price and gives me a known starting point.
Nev, the chain is endless with a riveted half link, Regina make so a branded chain. Have one on the VH and had no issues with adjusting that. I think others in the Club use the same chain (from John Budgen £13) and there has been no adverse feedback.
I agree re using the cam sprocket to set the timing, much easier than attempting to tighten the mag nut without altering the sprocket position.