Iain, You say that it does this on starting. Does it then stay like that whilst the engine is running, or does it revert to normal either after a few minutes or when you drive off?
Two possibilities come to mind: These instruments can be a bit voltage sensitive and as the car isn’t used much, could it be that battery volts are down, exacerbated by the electrical current consumed from starting?
Another thing I would try is to drop the dash board and check connection of all the wiring plugs, which can sometimes become loose, especially if they were not originally put on properly at the factory. (my loom behind the dash was an absolute mess!)
