On 6 July, 12:12, "Graham." wrote:

Disconnect them completely and use a multi-meter set to its
highest resistance range between L&E and N&E.

Use a high voltage resistance meter to check this, not just an average
multimeter. Don't complete electrical installation work without one.
If you don't have one, eBay's your friend.

For this sort of fault, a cheap DVM wil probably find the fault,
However I wouldn't "test" the system and claim it didn't have other
faults unless I'd tested the insulation at high voltage. You don't
need a few hundred quids' worth of 17th-ticketed Part-P-tricorder to
do this, just an old hand-cranker or a tenner's worth of cheap Robin
from eBay.