VW POLO 9N3 (2008) - Fuel Pump Sender Unit

[boobookowl]

I have just replaced a noisy fuel pump with an identical new unit. Pump is working correctly, but the fuel gauge is not registering on the instrument panel. Within seconds the petrol bowser light glows indicating empty tank, even though I know the tank is about three quarters full. I have rechecked to ensure the float mechanism attached to the pump is not caught up in any way and that the electrical wiring for the sender is still intact. Has anyone got an idea of what the cause of the failure might be? Thanks for any suggestions.
On 9 Jan 2016 gauge suddenly reacted (perhaps due to a bumpy road) but later in the trip the fault returned. If I now turn on the ignition the gauge needle will start to move from its resting position and travel to around the quarter full mark before returning to rest with the petrol bowser light coming on. Same situation after three more attempts.