Just got time before making my cup of tea (see above).
Tested the armature as per a nice easy to understand video and my simple multi meter. No faults found anywhere.
I'm going to put it down to the fact that the bush wearing out, after 8800 miles, allowed the rotor to move and touch the side plates, the wear marks and contamination on the rotor seem to back this up. Was the bush poor quality? Fan belt too tight putting strain on it? Don't know, won't worry about it (too much) either.
The rotor spins nicely, no movement in the bearing etc.
I will (eventually) lash out the 14€40 for a replacement bush when I place my order and rebuild the dynamo.
The 'revised' availability for the main item I want has now slipped to the beginning of July, it started off as being the end of May. My order list has doubled during this time!
Wow, perfect timing, off the make my tea!