Many thanks for your reply.
When I tested the output voltage, I did raise the engine speed to above 2000rpm, and it made no difference. The test procedure for the alternator says to raise the engine speed to 2000rpm, and measure the voltage across the battery terminals with the headlights on high beam. The voltage range given as good for this test is 13.9 - 15v. The test procedure then says to switch on the heater fan and the heated rear window and measure the voltage again; This time you're looking for at least 13.5v... I'm getting a full volt less than that with none of the above additional load. And it's this that leads to my original question:
In the 1996 charging circuit diagram (shown above), the 12v supply to activate the alternator comes directly from the ignition switch (via the BY wire), where in my car this wire is missing from the alternator completely; the voltage being supplied apparently from the engine ECU (via the WR wire): The question being, should I be getting a full 12v from the WR wire, and if I'm not, does it indicate a faulty ECU?