How do I check connectivity from Fp to pump terminal?
Set the multimeter to measure resistance (omhs)
connect one side to FP and one side to pump + terminal.
Use a long wire to get from pump positive terminal to the multimeter.
If the FP wiring is good you should get a very small or 0 resistance, infinite/high resistance means there is a broken wire somewhere.
 
Can check the pump ground connection the same way, 0 resistance to metal part of the chassis.
 
 

nissky thanks for your help with this so far. I did what you said and had mixed results. When I checked the pump ground connection to a ground point on the chassis the reading was 0.3 ohms which I think is ok.
 
I didn’t get a reading at all for FP to pump + terminal when I hooked up a long wire. This lead me back to the diagnostic port again. As stated in my earlier post, the circuit between Fp and +B on the diagnostic port read 2.4 volts rather than 12 volts. However, when I checked FP on its own with my line tester, the tester didn’t light up at all, no negative, no positive, nothing.
 
Any suggestions or explanations would be appreciated?

If you did it correctly, then it looks like you have a break on the wire going to fuel pump.
Also you shouldn't expect 12volts between fp and b, as fp is not a ground.
 
If you are defintely getting no voltage at fuel pump terminal under dash, with just ignition on and also with fp/b bridged.
Then I would run a direct 12volt feed from battery, and see if the car runs normally now.

Well I got to the bottom of it. The problem was the connector behind the passenger seat beneath the triangular window. Cant believe it was nothing to do with getting water or working in the engine bay. Thanks nissky, learnt a lot from this process.

Bugger, just when I was going to offer to take this (obviously) troublesome rally car off your hands.
 
Congrats on fixing the problem, though.

Thanks Dennis. You are going to have to let go of this obsession with my car!

Obsession is such a harsh word . . .          

hobby?