Mad Jock - I'm not an aircraft wiring expert, but I am an electronics engineer and have done a lot of electrical work on cars / motor bikes. I understand an aircraft wiring system to be closely related to an automotive one... so I've written this as if it were an automotive system. I may be wooofing up the incorrect arbour.
The Low Volts light will be on the regulator output to ensure that regulator is in 'charge' mode when the engine is running, as per the ignition light in a car. First test I would do to find out what's going on is to measure the battery voltage without the fan running and then with the fan running. I would expect 12V off line and 13.8V when charging. Turn all the electrics you can think of on - I would not expect the battery voltage to drop below around 13.3V with the engine running. If it does drop below that, then there is a fault in your charging circuit. I took your comment:
At all times the Ammeter shows current, jumps up etc if the nav lights are turned on
to mean that the ammeter is showing a discharge current, which increases with electrical load.
The symptoms of this would typically suggest a duff alternator / regulator, which have already been tried. One option may be to take the aeroplane to your nearest Lucas dealer / auto electrician and get them to give it a once over....
As you have a new one of each of these and ssuming that each is serviceable with good brushes, etc (is there a similar system that they can be put on to prove / deny servicability??), then in order of likelihood, I'd vote for the following:
- Poor connection between aircraft wiring and alternator / regulator
- As suggested by Monocock, chafed wiring in the loom
- Alternator field connection intermittent - check crimps, etc (if fitted)
- It's broke!
Hope this is of some help.
I would offer to fly up from Perth to offer moral support to your engineers, but our a/c is on yet another extended C of A / Annual and we aren't really sure when she'll be fit to fly again. Seriously, email / PM me if you want a contact number so that we can talk it through.