I just a volt meter on the standard wire. The thinner part that goes into the connector measure 0.5, and the thicker part measures 0.4 ohms.

I have got various replacement wire to use of different gauges, all bigger than the standard wire, but I'm confused....
Do I need to use the thicker gauge wire all the way to the terminal?

I=V/R, so it'll draw 12 amps at 12 volts. That's plenty. Either the wire is too small, or the power supply/traces on the control board are causing voltage drop.

No surprise there, it should read .9 to 1 ohm for decent performance, and even lower for max performance. You can easily draw 15 amps, however. Better have an awesome psu.

Depends on how well calibrated the ohmmeter is. What's the resistance shown when you touch the probes to each other, BFresh?

Is the bed faulty?

Is there anything I can do to improve performance?

Tested them both with a volt meter and both have a resistance of 1.5 ohms

Are there any software or firmware settings I should check?

If it is resistance and you used sufficiently sized wire, then the bed is defective.

If it's resistance, how do I get around that?

Thinking about it, I may have wired the terminals the wrong way round on the bed. In theory it shouldn't make a difference so I didn't worry about it too much at the time, but could it?

"old silicone heat pad between the alu bed"

This is probably not a good idea. More insulation the heat has to work its way through. Usually PCB beds replace the aluminum.

The old heat mat is on top of the alu bed, then the new heat bed, then glass printing surface. The old heat mat is there as a spacer between the alu bed and the new heat bed and has a cutout in it for the thermistor and cable.

Just to make it clear, the old heat mat is not between the new heat bed and printing surface so does not insulate the printing surface.

This is probably not a good idea. More insulation the heat has to work its way through. Usually PCB beds replace the aluminum.