OK whats going on here then ???

[John Mason]

I had this problem with my Ruby on the same dizzy and plug leads. It was put right by inserting plug lead suppressors In the four plug leads together with one in the king lead between coil and dizzy for good measure. They are not expensive and easy to fit. Just cut the lead and screw in to each cut end similar to how plug caps screw into leads. They are not unsightly and keep the original look with your brass plug lead ends. Another source of interference could be coming from your dynamo but I doubt it. If it is a 1mf capacitor attached between D terminal and earth should stop it. But try the plug leads first.
Look up Identifying and suppressing radio interference . howacarworks.com

John Mason