Right, my fellow Squarielisti, I have news.
First, the open circuit battery voltage is 6.2 V, on a bike that has not run for a week or two. I'm happy with that.
Second, removing the tank, rocker covers & HT cabling reveals that the tired rocker cover seals (no one should use fibre gaskets at those temperatures) have released enough oil to fill up the HT connections on the coil:
My bike does not have the rubber sock over the distributor...
Next, I know that these odd springs are correct for my 40426A distributor - what I doubt is that they are fitted in the correct holes, since the heavy spring never exerts any restraint on it's bob-weight (too much clearance when fitted in that hole). The smaller spring on the other side is always under tension, even as you see it in this picture:
The B5HS plugs are a good colour and have done a few hundred miles.
The capacitor looks good. Using a multimeter set to ohms, one can observe the resistance rise to some 25 MOhms over a period of a few seconds - a smaller capacitor (new) I have for a Lucas 25D4 distributor rises to a similar figure in much less time.
The coil also checks out - 10 MOhms or so to earth from the HT connection, and suitable low resistance in the primary; 10,000 x that in the secondary. I didn't record those readings, stupidly.
Obviously those are cold figures - they may be quite different when hot.
Lastly, some of the valve clearances seem tight; I have not checked the ignition timing yet.