There I was, hoping that JJ's diagnosis that Mmle was not raising any charge might be a mistake. Yesterday I took 2CV-doctor Llew over to see the old girl and to do a stethoscope impression with his multi meter. I'm afraid the diagnosis is confirmed. She fired up first turn, Bless her (better than my "normal" 2CV) but not an ounce of juice was getting from the dynamo to either the voltage regulator, or the battery.
This was confirmed as the battery voltage sat there ar a stubborn 5.6 volts no matter what - revving, headlights, you name it. Not even a flicker.
We didn't start taking bits off (yet), so we've no way of knowing whether this is a problem within the dynamo (we think not - the windings and commutator were replaced as part of the rebuild, and an expert said the bits all look fit off the vehicle.
There is hope though. Llew knows a couple of guys, coincidentally both called Kevin, who know about old car electrics, and specifically our 1961 6 volt system. Watch this space