John Rumm wrote :
To check the earth continuity you can disconnect the live at the circuit
breaker or fuse, and temporarily join it to the earth wire for the circuit.

Which would still leave the neutral connected and the neutral connected
to earth at the sub-station. One or more lights in the circuit left on
and the live will have at least some sort of connection to earth -
enough to cause some confusion when using that method.

The using a DMM on ohms range measure the live to earth resistance at each
lighting position (or switch). You should soon be able to find where in the
circuit you have a disconnection. Chances are it will be at a ceiling rose
somewhere - leaving the remainder of the circuit with a floating earth.

There were two standard methods to wire the lighting circuits using
T&E. The current method is from the Dis-board then lopped into each
ceiling rose in turn, then down from the ceiling rose to its associated
switch. The older method was similar to the above, but a joint box was
used as the central connection point for each section of the circuit.

If wired using the later joint box method it could well be that the
break in the earth is at one of these under floor joint boxes.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk