Thanks guys... the wall mounted thermostat I wish to replace is just a basic Honeywell T40, and the rads it controls all have individual thermostats. I had a quick look inside the T40 today, and it has a grey wire as well!
As it is already 'fed' from a three channel programmer, (combi boiler - hot water, underfloor and radiators) a 'smart' type of thermostat is probably a bit OTT.
All I really want is one which will display the current room temperature, with a visible indication of whether it is currently calling for heat, or not!
(I can see that a separate central heating thread is really needed here! )
Home and Garden has a Nest section.
should be able to replace the T40 with a Nest pretty easily.
the grey wire i presume is call for heat.
Earth, Live, Neutral, Call for heat.
in regards to wiring a nest.
earth can be terminated in a block and left
Live goes to L on Nest
Neutral goes to N on Nest
Live also goes to Terminal 2 on Nest
Grey (call for heat) goes to Terminal 3 on Nest