dodgy fuel gauge audi 100

[Coxy]

Got a dodgy fuel gauge/sender on my '87 100 sport.
Strange problem. When I turn the ignition, it beeps at me and the petrol warning light comes on and the computer trip display shows L for low fuel (I presume??).
Now, the thing is, I've just put in £25 of Esso's best, and the other thing is when the ignition is off, the fuel gauge shows what it should read instead of going to empty.

My first though is a dodgy earth on the plug that goes in to the top of the fuel tank. Have had the cover off and taken out the plug but nothing untoward, and no corrosion showing on any of the connectors.

Any ideas?