"xena" wrote
| No, there is no separate earth.
| The cable is bog standard 3-core flex, not twin and earth, so fully
sleeved.

This fails electrical safety standards on one, if not two, counts.

(1) Flex should not be used for fixed wiring. For some reason it often is by
people who call themselves Heating Engineers but whom I wouldn't trust to
wire up a rabbit snare.

(2) Whether or not the thermostat is double-insulated, all circuit cables
must have an earth (circuit protective conductor) running within or parallel
to them and an earth must be present at all accessory points.

As the house is up for sale it is possible that this would be flagged by a
surveyor and your overall electrical installation queried.

Possibly the best course of action would be to find the other end of this
flex (hopefully a 'wiring centre' near the boiler, in the airing cupboard,
etc), remove the flex and replace with a wireless thermostat.

Owain