Sometimes it is the simplest things that get you going (or stopped). One of my MK1s was doing odd things in that the generator and regulator seemed to be all over the place and sometimes not charging. One quiet afternoon I grabbed the digital multimeter and tools and attacked it.
The generator proved quite capable of putting out more than adequate volts and amps. This is a very simple test. Simply attach a piece of wire between the D [dynamo] and F [field] terminals on the generator and start the car. Don't over rev the engine, as 1500 to 2000 RPM should be adequate. Measure the voltage being generated at the generator D terminal. This will be typically in excess of 14 volts and possibly up to 16 or more volts. This is because you have bypassed the regulator. If the ammeter in the car shows a large output that also proves that the cutout is working. It pulls in when the generator volts are typically above 12 to 13.
All this was OK and I now suspected the regulator. I cleaned the regulator points but there was no change and I decided to replace it with a spare that I had. While undoing the terminals on the regulator I found that the screw for the wire running to the generator field was quite loose. Tightening this up was the answer and the generator now performed as Lucas and Jaguar advertised. Moral – look for the simple things first