Just been on a Honeywell course - recommend it thoroughly as it's useful and certainly not a straight sales pitch. Much respect! Bridges the gap between plumbers and electricians! My opinions after the course:-
The most cost effective thing is to keep what you've got running.
Personally I don't believe in Central Heating unless all of your property is occupied - however might be worth checking where your thermostat is (surprisingly - shouldn't be anywhere where there is secondary heating - often the hall is a good place - may seem wrong but makes sense if the system has been designed properly!)
Make sure you've got timer and thermostatic control.
Check the thermostat is connected properly (3C + E needed otherwise will bounce around on and off rather than a damped response).
Look to turn down your heating a bit.
Look into zoning. Mightily impressed with the idea of the new Honeywell system where thermostats on radiators are replaced with radio controlled thermostats. Allows zoning with no plumbing!
Hope this is of some use!