Bit more speculation . . . I wonder what does a meter show for battery voltage, across the terminals, with the engine running at various rpm? It should start at your 12.5v base or whatever, then rise to about 14 and settle there whatever the revs (or load).
Just wondering whether the thing is regulating properly.
If you run it with the battery disconnected and put a meter on the brown A line from the regulator to earth, what shows? Again, should settle at about 14v when running at 'regulated' rpm. Bit spikey maybe, especially with a digital meter, but should be good enough to show that things aren't heading ever-upwards. It should show much the same when lights are on, but need a few more revs to get there usually.
If the volt meter is like like a tacho, with voltage continuing to rise with revs and no sign of stopping them, then either there's a problem in the regulator or there is a wiring problem, as it would indicate the field input isn't being controlled. I have a friend's very pretty A7 in the shed at the moment, which exhibited similar high charge symptoms, until it stopped charging altogether. The dynamo is still good, but the solid state regulator isn't - I think the thing had effectively allowed F and D to remain connected (which is the correct situation between cut-in revs and when it should start fully regulating). Then it had cried 'enough' and died.
Normally, as mikeb says, you'd see a small steady charge, with blips of more after, say, idling with the lights on for a bit (as the battery gets a boost to compensate for the temporary drain). With a fully-charged battery, half an amp charge would be pretty normal as a cruising reading out on the road.
The risk (if the thing doesn't settle down soon) is that the dynamo could be 'over-served' like happens to some of us now and then down the pub, and the system could get itself a nasty hangover.
