Now then chasps, and 9 or 10 laydeez, it appears to be fixed! It wusnae a dodgy ammeter, nor a foosty ignition switch, but, can ee believe it, the ignition wire had been rubbing on the exhoost and a toaty wee bit o bare wire was showing thru and making an intermittent earth.
The autospark also rewired the lucas plug to make sure. We huv dun aboot 30 miles so far withoot seein the charge light.
Now I hear Charles say - he cud have fixed that himsel, and I'm sure I cud, had I spotted it. But I didnae. I might do annuther time tho.