The direction indication would only have been reasonably accurate for an hour or so before resetting due to gyroscopic precession (caused by drag on the gyro bearings among other things) but this would normally be enough for most purposes. In a steel vehicle it would be almost impossible to fit a remote magnetic compass (as in aircraft) to feed continuous updates to the gyro to correct the drift.
Where they got their correction from I can only guess - the most obvious is for someone to stand well back from the metal mass of the vehicle with a prismatic compass and call out the centreline direction to be wound on to the indicator. If you were really stuck inside for long periods an educated guess could be taken from experience with that particular instrument to wind on 10 degrees, or whatever, East or West every hour.
The beauty of a gyro system is you can put anything you want on the dial so you can have true bearings to read straight off the map without worrying about magnetic variation calculations.
I think they were probably meant for circumstances such as an assault on a pitch black night or if you needed to be closed up (Churchill?) for comparatively short periods.
Lang
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