I'm curious, if you do have an earth fault on the lighting circuit
(lets say the earth goes back to the consumer unit earth bar). If
nothing trips how will anyone know that the fault has occurred. Surely
it can't run like that indefinitely?

If I can't run a new cable back to the CU is it better to have no earth
at all on the lighting circuit or to take it from the socket circuit? I
suppose I could take an earth off the lighting circuit now and connect
it to the CU whenever I renovate that area of the house (may be years
from now). I suppose plastic switches would be required until I get it
properly earthed.