I have at last got the bike sorted. Many thanks to various people who sent me instruction manuals.
The commutator was a bit dirty. Once polished, the dynamo motored, and gave lots of volts on the bike.
The major fuse blowing was a short at the new pattern ammeter which I fitted during the time I have been trying to fix the electrics.... I carefully re-installed it with wires pointing in better directions, and covering everything with insulation tape to prevent any shorting if wires did touch things.
The CVC unit was giving around 9.3V. Thanks to instructions sent to me, I was able to adjust that down to 7.9V
People who advised me just to throw the Lucas CVC unit are probably right, but I have now had the satisfaction of sorting out the original system. I can fit an electronic regulator when I feel ready, instead of having to buy one URGENTLY to get the beast back on the road.
I was intending to go to the National Rally in Gloucestershire, and then the Hever, but had to sort the bike out first. Well, at least it's done now.
Paul