If your boiler is the Servowarm Elite HE 50 it was made between 1993 and 2000 so will be between 9 and 16 years old. This boiler has quite good 85% efficiency.
Not having a room stat will be both inconvenient and unnecessarily expensive for you in fuel bills. Have one fitted.
Engage the services of an independent boiler engineer to do the work - avoid the larger organisations who will charge you considerably more - have the boiler serviced when the work is being done and consider having TRV valves fitted to your radiators too at about £50 per radiator.
Good luck...
Peccavi
October 2009