Thanks for the replies. I have some follow-up questions.

One poster said to tie my earthing requirements back into the main
earthing system.

The cabling we are talking about here is all in the loft of the house
and I do not recall seeing any earthing cabling up in the loft at all.
The lighting circuits do not have any earth wires and there are no
power sockets in the loft. Only a single lighting circuit.

Should I look further for an earting system up there. This is a
1930's house with a wiring system from that era, although most cables
are of the newer (white PVC) type I suspect the wiring layout design
is of the 1930 era.