My gas pipe is definitely straight in the ground, as I can see it - its an ancient iron/steel thing that disappears into the earth. Anyway, I disconnected the bond from the gas pipe and still got 0.46, but then it ocurred to me: as the pipe is connected to appliances which have their metalwork earthed, am I not getting a parallel path through their CPC's? I'm not going to the trouble of disconnecting those, or the bonds on the water pipe or central heating as they are under the floor and a pain in the arse to get to. I'm getting more confused about this by the minute!
The more I think about it I think it is the gas service, as (a) it is a metal pipe going straight into the ground, and (b) the gas-main in the road goes right past the transformer within about 2 metres of it.
Testing the MET without the twig got me 0.35.