My BG fuel gauge drops to zero when I have half a tank still left.

Anyone had that problem, was it the gauge or the sender unit?

probably the sender unit in the fuel tank. The gauge in your dashboard will receive only a signal from the sender unit.

You can take the whole fuel pump out of the tank and see whats what.

Yeah agree with the above. Take the whole unit out and see whether everything is in order and as it should be. Might be a slight connection issue or blockage somewhere?