I'm a bit stumped and although the fault seems to have gone I'm nervous about it returning as I haven't found a root-cause.
A month or so ago I got the Spitfire back on the road and started running in the new engine. But over time I found the ignition was too far advanced giving me pinging even under moderate load. I re-timed everything on the assumption that either I'd done the timing wrong in the first place OR that things were just "bedding in". After a week or so of running I checked it again and this time it's way to far retarded, so I re-timed again. Then the other night it was so far out that the mis-fire stopped me in my tracks, "twiddling" with the dizzy position got it running well enough to get me home. I've gone back to my old dizzy now just to be on the safe side.
I pulled the water pump and took the timing cover off to check that the vernier gear on the cam hadn't moved - still where it should be and no signs of movement (all the bolts re-tightened and thread lock applied)
I've inspected the recon dizzy I removed and I can't find anything wrong with it, because of the engine spec there's no vac advance and the base-plate is firmly locked in place. The bob-weights seem to move freely as expected. The recon company have (very kindly!) agreed to re-inspect the dizzy if I send it back to them - so at some point I'll do that.
Dizzy cap is locking firmly in place with no slop/movement when clamped down.
I'm out of ideas if the cam timing isn't moving and everything in the dizzy seems to be behaving I can't see how my timing is all over the place. Oh and I've see the same "jumps" and "moves" in the timing on both points AND electronic points replacements.