We had the radiator installed at the time of construction. It wasn't too difficult a job so it might be worth considering. How large is the conservatory? have you thought about a portable oil filled electric heater. They look like a normal radiator and chuck out a lot of heat. not pretty though.

I didn't think log burners created condensation but I don't have experience of them. How/where would you put the flue/chimney though?

Sbg is right, any apparatus involving burning is going to require ventilation to the outside, and that will eliminate condensation--from the heater, anyway! Wood burning stoves will require a full-up vertical chimney, to reduce soot and creosote build up, while high efficiency gas heaters can be more flexible in their venting arrangements. If the conservatory is small enough, an electric space heater, with due attention to safety procedures, may be the best alternative. That should be enough to raise the temps those critical few degrees to allow half-hardy plants to thrive. Most of the condensation will result from evaporation from the plant leaves, and the difference between indoor and outdoor temps.

If by a 'log burner' you mean a wood stove then 'no' it doesn't create condensation but I can't see how you get the flue out through the glass. Is it really impractical to extend the heating system to have a radiator in?

We have a wood/multifuel stove in the sitting room and it produces a dry heat (it does have a chimney to the outside). I don't know how large this conservatory is but even a small woodburner is likely to produce a lot more heat than a fan heater. If you just want to light it each time you will be sitting in the conservatory then it should be fine but if you want a steady heat for tender plants then I would be thinky electric with a thermostat.

Hi, many years ago I had a large wooden greenhouse, and the heat was provided by a parkray closed coke fire, I had to get a metal fabricated fitment to go over the square outlet at the back of the fire, with a circular piece of metal which connected to a circular asbestos pipe which I used as a chimney, I replaced a pane of glass with a piece of sheet asbestos with a hole cut out, and attatched an "L" shaped piece of pipe to prevent rain going straight down to the fire, I then added 2 radiators, with a water supply tank to keep the radiators topped up, this kept the greenhouse nice and warm, even in the coldest winters, although I don't think this method would be possible these days, asbestos has been banned for years, you would probably find it difficult to get the fire, and wouldn't look very nice for a conservatory, but it was ok for a greenhouse, Derek.

I agree with Sbg...We also had a radiator installed in our Conservatory ,from our Gas Central Heating system,but although it is the correct size in terms of heat output, for the size of the conservatory,we never found it to be warm enough to sit in on cold days,so we bought an oil filled Radiator ,to plug in when and if it is needed This proved to be much more efficient,with a variable heat control..I really wouldn't go to the expense of a CH radiator ,when a portable one will do an excellent job. .providing you have a plug socket,of course..ours lives discreetly behind the Conservatory settee,when not in use..My plants overwinter successfully in a cold environment ,as we only use the heater in spring or Autumn,when we wish to sit in there,or on cooler 'summer' evenings..We have never had condensation on the glass,as it doesn't get extreme changes in temps,except in summer,but we have 'trickle' vents in all the opening windows,so even when locked,we still get a little air circulating in cold weather,which helps prevent it..good luck,whatever you decide to do...

Thanks to all of you for responding and your input. The conservatory is 12' 6"x 10' 6", so it is quite large and it wouldn't be practical to add a radiator off the central heating, how the rooms are situated.
It is south facing and on sunny winter days the temperature can warm up by the afternoon, to sit in there for a few hours, without needing any heating. We use it as our dining room in the warmer weather and plug in an electric oil filled radiator and have an additional fan heater, if we need to use it in the winter when we have guests. Most of the time it is unheated and the plants that seem to thrive out there are Aloe Vera, they are constantly growing and producing babies!!!
Searching on various sites, it looks like a wood burner would be possible, so before getting a company round for quotes, I was just interested to know if any members had had one fitted and what problems might have been encountered. I will let you know if we do decide to go ahead and have one installed, thanks once again for your replies.

An interesting side issue re CH radiator in case this might be useful to anybody else - we had a structural problem in our conservatory recently and the fact that we had a CH radiator in there it made a large difference to how we were charged - I can't remember the details unfortunately but it was something about the presence of the radiator making the room legally part of the house rather than an add on. (It was only a short run of piping from the one in the dining room so had not been a huge job to install it)
Before we had the radiator we found a small fan heater on a frostat was all we needed as in winter we only sit in it on nice sunny days when the sun warms it quickly.

If your installer can fit a flue then a woodburner would be an excellent choise for your use of the conservatory.