Erratic Fuel Gauge

I have a 1998 A6 2.8 Quattro Avant ( uk car) with 150,000 miles in very good
condition & full history. Lately the fuel gauge has been playing up, showing
full for 150+ miles then dropping rapidly until it gets to half, if I then
fill it up I can get 50-55 litres in (70 litre tank) so obviously something
is wrong. I have removed the tank cover and operated the float by hand and
the gauge does not go below half even when on the bottom of its travel,
anyone got any suggestions as to whether its the tank unit or the gauge, or
possibly a voltage regulator somewhere, (everything else works fine). If its
the tank unit should I try a second-hand one as I imagine that to by a new
Audi part means taking out another mortgage to pay for it.

Any help would be appreciated

Graham