Hmm, that's interesting. My house is a 30's place, but the wiring
looks reasonably modern to me (circuit breakers not fuses, etc.) Just
how old would wiring be for you to expect lighting circuits to not be
earthed?

It is very easy to replace a consumer unit and accessories. (I just replaced
my old fuse box with a modern consumer unit with RCBOs etc. in 3 hours). It
is very difficult, expensive, disruptive and time consuming to replace the
cabling. You will, therefore, find that consumer units and accessories do
not give a good indication of the age of the wiring as they are frequently
upgraded to keep up with developments. (i.e. old fuse box replaced with
MCBs, then an RCD or RCBOs are added at various times).

Basically, if the lighting circuit has no earth, it is probably pre-1970. If
it is made of rubber, instead of plastic, it is probably pre-1965. These are
not precise dates, however.

Christian.