Lack of fuel when car gets below half a tank

[Ianhorsham]

I have replaced the right fuel tank sender as gauge was dropping off. Now when I go below half a tank my glow plug light comes on and lack of power. Indicating low fuel pressure. If I fill up again all is well but only takes £70 so tank not empty. This has only happened since sender replaced. What have I done wrong?? 

[niedzial1983]

Get vcds scan to check for faults will be best start