2005 Renault Megane fuel gauge reads high when ign on - but shows fuel level when ign off! display shows correct miles remaining so tank sender must be ok?

When the ignition is switched on the needle shoots right to the top (above the full mark). When switched off - it comes down and appears to show correct fuel level (though it should drop to 'E') The computer readout shows the correct number of miles remaining in the tank - and counts down as you are driving - thus I am assumeing the tank sender unit is ok. Is this likely to be the dashboard gauge itself - or something in a 'module' somewhere?!