I noticed that a light switch back box measured around 100V ac w.r.t. the
cold water tap in our recently-purchased house.

Thanks everyone for your advice and a few fault-finding pointers. I did
quickly realise that the voltage measured was probably sourced from a high
impedance and that measuring with a DVM with a 20M input Z would giuve
unrealistic readings - hence my decision to measure the current and see it
as only a few hundred uA i.e. a "leakage" current.

I'll start working round the ring - sorry, the radial wiring, thanks for
putting me straight - and we'll see where we get to. Hopefully I'll find a
floating "ned" which can then be tied down.

Thanks all.

DDS