I have 12 v power going to the switch and I have purchased a new bezel (including switch) and the pump functions perfectly (when connected directly to the battery.
The problem is that when a load is applied, i.e. the pump, the voltage reduces to 0v and the pump will not operate.
Even if I connect the pump directly to the 12v feed before the 3a fuse the same thing happens. I am in despair!!

Chris, July 2015

The pump is good and the switch sounds to be good from what you say.

I strongly suspect a wiring error - I can imagine both +ve and -ve both being wired to a single pole switch

Draw the circuit on the back of an envelope and see if you can make it work in your mind.

Supply - switch - load (pump) - back to the battery to complete the circuit.

You do not say where the zero volts is being measured.

Good luck..., July 2015