The only thing I was still open to was any voltage on the N from the supply side of the switch (not the load) as that would still be a function of the resistance of the neutral in the supply (wouldn't it)? Not if the RCD / switch was open of course but if someone was trying to close it?
Or maybe it does discriminate between the input and output (of the RCD) re it's ability to trip due to an imbalance (maybe it would have to)?
Cheers, T i m