Hi,
@TomGeorge, GND to SOURCE voltage (directly on the pin) is 0.4v when om, 0.0v when off. With the BATT + () connected to the MOSFET gate and resistor removed, I'm getting 1.0V
Your source to gnd wire is faulty or protoboard connection not good, the drop should be zero in both cases, because source is connected to gnd.
This will take 0.4V of gate level away from your MOSFET.
Tom.. 