Finally got around to removing the alternator. As several members thought the culprit might be the regulator I've posted a pic to show the wear.
I've also taken a picture of the alternator rotor.
I must admit I don't know what "normal" looks like but I believe rounded pickups on the regulator and groves on the rotor mean a new alternator (with new built in regulator) is in order.
Does anyone second the motion?