2000 Renault Espace fuel gauge not working

hi there.the fuel gauge on my reno espace is faulty.what happens is, if i put £20 of petrol in. when i turn the ignition on i get 3 bars on the gauge for a few seconds. then it shows 1 bar. but after about a mile, it beeps then shows nothing. then the service light and fuel pump flash. i have changed the sender unit but as not stopped the problem. can someone please help me. many thanks gaz.