Fuel Gauge Has Mind Of its Own

Hi I recently purchased a 1998 Ulysse 1.9 TDS (R) and I have a question about a Fuel Gauge problem.

I put £50 of diesel in it when I collected the car which gave me a little over half a tank. The needle stayed there for ages and being an ex HGT owner I was chuffed about my new fuel economy. I noticed an issue after 200 miles and the needle still had hardly moved. Now when I start the car I seem to get a correct reading, then after a time the needle will begin to move up towards the half a tank position.

Before I collected the car from the second hand dealer it had been sent to Fiat for a separate problem with starting, which was finally diagnosed as the fuel pump. While investigating the problem the car was put on their computer and it showed no faults. So now I am a little confused as to what could be causing this.

Basically is this a common issue on the Ulysse and if so is there a common fix.

Last thing I want is a dodgy fuel gauge reading.

Hi, I had problem with fuel gauge earlier this year on a 04 2.2 jtd. Fault didn't show on the onboard diagnostics, or dealers computer. Only found problem when they took the fuel tank off. It was empty, gauge was reading 1/4 full. Sorted it by software update. Cost £140 ish, including 80l of diesel.
Hope this is some help, good luck.