Thanks everybody. Yes I am in the UK. I guessed there should be a dial tone on the line (I switched to cable years ago) and I also think there are 3 lines coming into the house.
I have now found two (very old) master sockets (no internal test sockets) on which the phones work ok but the modem will not connect, this just leaves the mystery dead adsl socket which should be connected to somewhere, but I don't know where and it is going to mean removing large parts of a large house to find out.
Edit: I am with Virgin myself, this whole house demolishing episode is for my mate who calls me whenever anything goes wrong :-?