Nigel,
I think that the following is the typical scheme of the power connection of a house. If the connections are well done, the "earth points" E1 and E2 should be at the same potential because earth itself is a very good conductor so neutral and "protection earth wire" are almost the same wire (from very few tenth to very few units of ohms depending if ground is wet or dry) so I think it will be impossible to use them as wires for a communication. Am I wrong?