mazda 3 fuel gauge stopped working?

[danneth]

Went to the garage today trip computer was showing 17 miles remaining, put £20 in drove off and thought hold on a minute gauge hasnt moved trip computer was showing 16 miles, went back to the garage as i thought it might be a problem with the pump, no problem with the pump went to a different garage put another £10 in gauge didn't move but trip was now showing 18 miles, anyway ive drove home about 6 miles and its slowly creeped up to 40miles remaining but the gauge has stayed the same

any ideas??