O
Octopus
PS I think Grommit you may be onto something with the different earth potentials, one of the things I'm taking with me is a stake and once I've checked for all other faults, I had thought just to take L+N from the house, use two RCD spurs and they can run as a TT system down by the pool.
Lucien seems to have written out the capacitance idea, grabbing at straws really, so two different potentials does seem the most logical way to look at it.
Did you check the continuity of ALL the cores and the wire armour before doing the IR?
Too many people do R1 + R2 , but few seem to do Rn + R2 ...... which can be very revealing