Alternator not charging

[mgtf328]

New (to me!) Healey (BJ8) and yesterday the Ignition Warning light wouldn't go out. It has an alternator by a previous owner. I tested at the battery - no charging voltage.

I got a local shop to test the alternator - nothing wrong with it.
The brown/yellow alternator wire and all the brown wires that went to the old Control Box are all connected to a green wire which is routed through the bulkhead to behind the dash. I haven't been able to find out what the green wire is connected to yet despite nearly getting stuck upside down in the passenger well but I suspect it's the ignition switch. Anyway getting nowhere I put the alternator back and now the light goes out as before and it seems as if there's nothing wrong!

So what went wrong and will it happen again?

I just finished charging the battery (12.9V) and will check the alternator charging voltage at the battery again tomorrow.
However, I've been doing a little research in the meantime. The warning light is an LED and I've read somewhere this circuit needs a resistance in parallel to increase the current sufficiently to enable it to energize the alternator. Perhaps the resistor has blown and there's enough voltage to balance the warning light but not enough to charge the battery. I've no idea where the resistor is located. Can anyone shed any light on this theory or am I talking rubbish?? Wouldn't be the first time.
Another point that occurred to me is that I noticed on going through the paperwork I got with the car an advisory on a service invoice saying that the ignition switch may be faulty. Could that be the cause?
If I still find no or a low charging voltage again tomorrow I'm tempted to connect an old fashioned bulb or perhaps even a resistor in series with the indicator circuit see if it increases the charging voltage at the battery. Any comments/advice would be gratefully appreciated.

Thanks

Aj