Hi Peter,
Going from memory, I believe that many ECUs hold the lambda sensor earth slightly above true ground as a means of detecting shorts to ground on this wire. I am not sure if Ford use this technique or not. It is also almost a certainty that the lambda sensor earth comes via the ECU for the purposes of accuracy of measurement and noise rejection. I would check the earth at the ECU (PCM I suppose as it's a Ford, but old habits die hard.) If it is not a broken wire it is possible that the earth track in the ECU has blown up due to a fault current overloading it or water ingress into the ECU plug.
HTH Shaun