I think I need to add a word of caution before everyone starts pulling their instruments out the way I did...
Here's the procedure as I have been instructed by Roy Gillard of MECUK:
1. Release the two screws under the heater arrangement.
2. Release the screw on the left side of the heater arrangement (towards the instruments).
The reason behind taking the heater panel screws out is that the left hand side of the instrument panel is just tucked slightly behind the main dashboard moulding and you need to move it to the right, otherwise as you push it out (and even more so when you come to push it back) it will damage the soft aluminium instrument surround. With the heater panel loose, you can move the instrument panel slightly to the right and the screw head on the side will no longer get in the way.
When you push it back in make sure the left hand edge is not fouling the moulding, and once it is down you can move it to the left slightly.
As for the accuracy of the battery indicator, as I said in another thread, it is fine IF all the connections are clean and tight, but often you can lose as much as 2 volts between the true reading and the connection at the back of the gauge! This is why it always reads low. So you need to check, clean and re-tension each connection, particularly those at the circuit board, and when you have, the gauge will be near to, if not correct, and be more useful.
You should understand that this is only an indicator. It is not a true voltmeter. In fact it is a short scale damped meter, meaning that it only reads between about 10 volts and 17 volts and owing to the damping is also slow to react and therefore shows only steady readings properly. It is fine for normal car use where the charging voltage must be 13.4 to 14.4 volts. So if it is above 14.4 it is overcharging, and below 13.4 it is not charging sufficiently. Therefore the gauge (which has no actual figures - only coloured sectors) only needs to show around 10 - 11 volts to show a low battery, around 12 - 13 volts for a good battery, 13 - 15 volts for charging and anything above 15 for overcharging (all figures approx. of course).
It is a normal bi-metallic strip instument as Anders has pointed out, and the strip curves and makes the needle move in proportion to the current going through the windings. Normal external heat will not affect it so normal interior and dashboard heat is insignificant. You shoould not be bending or altering anything inside, without first having correct voltages and earth at the terminals. It was originally calibrated for correct voltages and once you have these, it will give a good indication.
Once I cleaned and corrected all mine, the gauge read fine. It sits very slightly right of centre when charging, left of centre for battery level (engine not running) and only drops heavily to the left if not charging correctly.
One final thing - the original circuit boards used aluminium strips and circuitry, which is never good for electrics on a car, but later replacements (I have had several now for different cars) are much better quality with a different gold coloured material. These give better results.
Roy