Those of you who offered suggestions about my intermittent high charging voltage on my PI might be interested in the outcome.
Round 1: First surprise was the number on the alternator - 18ACR when I had expected to find 17ACR or perhaps 15ACR. There were 3 wires from the back, a thick brown, a thin brown and brown with a yellow stripe. Inside the alternator the connections for the two browns were riveted together, so I was obviously looking at the brown/yellow connection to regulate the voltage. According to the wiring diagram, this wire feeds the warning light, and as the light was OK, I didn't reckon it could be open circuit.
Inside the alternator was a bit oily, and the were internal connectors from the regulator with an oily dead leaf stuck between them. So I removed the dead leaf, and degreased everything with Coldkleen, cleaned up all the blades that connectors attached to, and reassembled. Instead of an intermittent fault, I now had a hard fault - 16V whenever the engine was running.
Round 2: Actually test the brown/yellow wire is not open circuit - seemed OK. Took a bright light and magnifying glass to plastic connector block, and noticed that the brown/yellow wire was further back in the block than the two brown ones. Worked out that the back of connector block prized off and removed it. Pushed connector block onto alternator and noticed that brown wires stayed put but brown/yellow wire moved back, and came out when I gave it a tug. Shoved it back in its slot and pushed it right home with the aid of a screwdriver blade, and when I started the engine, 13V :-). Clipped the back of the connector block on without disturbing anything and job done!
Thanks for the suggestions on where to start looking.
Jim