Its more difficult than it sounds imho Davo. Get it wrong and you will have a dead short high current fire in the cabin

So you need to be careful. Voltmeters are easy and relatively safe to fit, anyone can do them, but voltmeters on their own don't tell the whole story. A diode gone in the alternator will provide a little bit of charge only which won't be apparent with a voltmeter but will elave you dead when the lights go on for a drive at night. Lucas and other non-shunt Ammeters are another issue as to be accurate every stitch of current goes through the gauge which brings large gauge cables and high currents into the cabin..
Placement is everything. If you put it between the alternator and the battery you will see the charge current out of the alternator, but have no idea of what Amperes the car is drawing in total.
Put it in between the battery and the car's accessories and you will see the current drain, but have no idea of how much the alternator is providing.
To be truly useful showing either battery drain or battery charge one needs to create a bottleneck in the wiring where all the charge from the alternator passes on its way to the battery, and all the charge from the battery to the car's accessories also passes through in the opposite direction. Coupled with a voltmeter two gauges will allow you to diagnose everything the charging system is doing.
For wiring systems not designed to have a ammeter fitted it means you need to come up with a educated wiring plan, and create a current bottleneck to mount the ammeter. Then you need to make sure the terminations are created with a professional finish as a failure here will at the least have you sitting on the side of the road, and at the worst cause a full current dead short and fire. Not easy on a congested Rover bay.
I don't mean to put you off, but fitting ammeters is tricky stuff to do properly. Do any less and you will only have the gauge moving one way or the other in positive or negative directions. It is best if you get an autospark to fit it up. I would do it if I was still down there however Davo...