Please can someone just advise me below?
While trying to work out what's going wrong with the 3way valve on our
central heating, I've been fiddling with the hot water cylinder thermostat a
lot, and it's become stiff. (no tittering...)

Is it a simple job to replace it, eg 2 or 3 wires, standard codes?

Current one looks like
http://www.toolstation.com/search.html?searchstr=47346

and has a single white cable going into a 10-way honeywell connection box
in the airing cupboard.

The thermostat cable has 3 wires, red, black & grey.

regards