We don't bother with anything through the summer. We leave the doors open between the conservatory and dining room all the time, but bought a 3kw electric convector heater (about �40 from Argos I think) which has a thermostat on it. Very rarely have it cranked all the way up. During winter we normally leave it on about 1/4 power which keeps the conservatory at a comfortable temp, and of course you can always turn it up if you want.
If you definately only want background heating then the best form is underfloor heating. Its easy to install, works off both a thermostat and a timer and is a lot more cost efficient that the previously mentioned convector heater. The only problem being of course is whether you've already fitted your floor. If not I really recommend this type of heating. It can be laid under any floor covering, even carpet.
Fit an air conditioning heat pump. Much cheaper than gas or electric fires to run, you will make your money back after 2-3 years. Try www.global-cooling.co.uk for some ideas or costing.