In message .com, Ian
Middleton writes
Ah ha. The earth wire has been found, it just started about 1-2"
further down the wire than the live and neutral. Interestingly the
wiring diagrams show a single black line connected to the thermostat,
not showing it is really 3 wires. Anyway L/N - earth measured at
greater than 50M both before and after instalation so all OK.

If the earth wasn't in use before it might be worth checking it's
connected at the other end. In the hands of idiots I've seen the earth
core used as feed in a two way lighting arrangement. Never assume
anything. That's how you get electrocuted.

--
Clive Mitchell
http://www.bigclive.com