Thank for your reply, I will have to check on the earth situation, I had an original RST loom and plugged that in to use. So I would have though how it was meant to be set up when standard. Am I right in thinking that using the second earth makes the computer measure fuel flow from just the sender and speed sensors? Ignoring the fuel flow sensor?
The Cosworth engine management is not linked to the little loom at all, only the in-line fuel flow sensor, speed sensor, fuel sender (in the tank) and the main power are connected to it. Although I have seen some threads where they have used an output from an ECU to measure the fuel flow through the injectors.. I too am not sure if that would work.
Would be nice if I could get it working or at least reading more than 3 MPG

Cheers for your help!