Get the wire so it's thick enough to carry the current and not cause a voltage drop - if you get wire rated at 30A, then this should be up to the job.

Run the new wire directly from the battery (with a new 30A fuse near the battery) to pin 30 of a new 30A relay in the boot area. Run the same size wire from pin 87 of the new relay to the pump +ve terminal. Run the same size wire from the pump -ve terminal to a good earth. Use the current wiring that runs to the pump +ve terminal as the switching wire on the new relay (pin 85)(using the original wiring to switch the new relay means that you are still utilising the ECU to switch off the fuel pump in the event of an "incident" and also the kill switch in the spare wheel well). Finally, run a smallish wire from relay pin 86 to earth.

After rewiring, you should get well over 12V at the pump at idle.

Don't forget to have a look at the fuseable links at the battery - corrosion of these is a common problem.