There is an RCD in the Consumer Unit (see pic). I'm assuming it's protecting the 5 circuits to the left of it but not the 2 lighting circuits to the right?Is the circuit protected by a 30mA RCD ? If the answer is yes then the results are pretty un-inportant. Maximum Zs is 1666 ohms. In practise a reading over 200 ohms woukd be considered unstable. Sorry if I've confused things. By the way if it has no 30mA RCD protection then that would be more of a concern.