I've recently had my gas boiler serviced which pulled up a fault on my upstairs mains cct. The cct has no earth connection to it. Pulling back the socket fascias shows that all the sockets have a good earth connection to them so to check I ran an extension lead from the downstairs cct and plugged a plug in without the back on to expose the terminals. Metering between the exposed earth terminal in the extension lead and a earth wire in the back of one of the upstairs sockets showed six meg resistance. Besides the obvious, making sure it has a good earthing at the consumer unit and breaks in the cable, does anyone have any other clues as to what may be at fault?