Good news- Stack voltmeter is in, so I can keep an eye on things.
Bad news - Voltage is pretty low, resting at a fluctuating 12V when driving with the headlights on.
Drops below 12V if I'm parked up (engine running) with lights, heater and wipers on, down to about 11.5V.
Some of that will, I guess, be a result of me tinkering with the gauges earlier (switching the ignition on and off a lot) but it's done around 100 miles since so should be fully charged?
The alternator was changed about three years ago, so should still be healthy. Battery is only a year old, so again that should be fine.
Earthing problem?