I once accidentally put the battery leads on the wrong terminals and, Mr Clueless explained, this could lead to the dynamo reversing its polarity. So I followed his instructions on flashing the dynamo, and five minutes later, the ammeter was back to life and the battery taking a charge.
After this success I fitted the 6v LED headlight and stop/tail light bulbs that Crazyfrog mentioned a while ago and wow, we have light. Very bright, and they use so little electricity. I don't ride at night, so even if the beam isn't focused well (and it seems OK) it'll be very useful to make sure I'm seen in poor daylight.
With lights on full beam, the ammeter used to read about 6, but with the LEDs it's hardly anything.
Results all round! And for an electrical ignoramus like me, that's a real achievement. Time for a pint of Adnams.