Thanks TurboShed & Tabetha. I was about to remake the engine earth but it started raining! I did disconnect the earth in the boot with the engine running though - and as I pulled the cable away there were some sparks (just before the engine died!). So there's a few amps going through it. Funnily enough, not 600mm away is another earth from the other rear light cluster which was tight. Next time I'll disconnect that one and see what happens.
Sorry it's only half the story but every time I go near the car it starts raining. Do you think God doesn't like Fords?!