Thanks for all your replies, everyone. The slide is a 3 1/2, Klaus. It also accelerates fine - right up to 70. Then, when I drop to a steady 60 it happens after 1 or 2 k. The whole set up has performed wonderfully, for 4.5k miles (since my top-end rebuild, new carby etc). I think the timing is good - because it was pinging a bit last year due to points gap too wide. Now it's back to about 12 thou the pinging has gone away, and it otherwise performs great.The only area I haven't eliminated, I believe, in this recent journey of fault-diagnosis (recently shared with you all), is, yes - magneto/ignition circuit (but I have new plugs). To even start looking at any of the factors involved around that device - I expect to, therefore, bite the bullet and take it out. I would deliver it all to our fantastic local engineer then, in the first place. I have something else for him to do. He can check everything, I don't have to go through things one by one.
So, I might not report back for a while now - thanks for everyone's forbearance..