Check how many wires go to the existing thermostat.
Most of the old mechanical ones only have 2 wires, because they are just a simple switch. If that's the case replacing it with a digital controller will require a complete rewire of the control system. You have to supply operating power and control circuits so it could work out quite expensive.
The alternative would be one which operates via radio signals and powered by batteries.
http://cgi.ebay.co.uk/RADIO-FREQUENC...QQcmdZViewItem
The cost goes up if you want a full control system.
http://www.syxthsense.co.uk/ecommerc...bcategoryID=78