Just a note to those considering buying a programmable thermostat and deciding on the make to buy.
I've just bought the Sunvic one from B&Q. At 30 quid, it is much cheaper than a Drayton Digistat 3. Rather than programming with the normal 3 on/off times per day, you can set up a heating profile with 24 dots representing the hours and have complete freedom to set the dots (i.e. up to 12 periods per day, if you are weird). You then set each day to the profile previously set up. (There are 6 factory suggested programmes too). You can set each day separately, or set 5/2 or 7 days all at the same time as you wish. You can set both the 'on' and 'off' temperatures. It has logic to pre-empt the onset of an 'on' period (but no external temperature sensor).
It displays the the time, call for heat, the day, the temperature and the 24 dot profile at all times. Although slightly bigger than the standard light switch size, it only sticks out of the wall by a relatively small amount and looks very classy. Much less intrusive than other makes. It also has 6 buttons, making programming much easier than those with fewer buttons. It even has a electroluminescent backlight and manual override (both programme advance and temperature adjust).
In use, it keeps temperature well within 1 degree at all times. Very highly recommended and much better than the Drayton Digistat 3, IMHO.
Christian.