Invested in a new BTH electronic magneto in place of Lucas points unit.
Discovered that the original sprocket from the points unit is too close to the mag face and doesn't align with the drive sprocket.
This is the solution. You need a magneto sprocket from a 1948-1958 model ( Norton ES2 or Model 50) before they changed to alternators and points. Seems obvious now but took a while to find that out- thanks to Mike Pemberton !