Hi Buntyboos,
In my own experience, and having kept a greenhouse for fifteen years or so now, my advice would be to save your time, and more importantly your money, for something more worthwhile.
Thing is, greenhouses are really not suited for heating. 3mm glass panels overlapped and unsealed would be just about the last rational choice of building material if they were.
Greenhouses by their very nature 'breathe', as it were. This allows a constant exchange of vital gases and the removal of water vapour, even when doors and vents are sealed.
Seal up a greenhouse and watch how soon it is literally dripping with moisture from both condensation on the glass and vapour transpired by the plants themselves.
Or, easier than sealing a whole greenhouse, just pop a plastic bag over a plant for a while and see how much water condenses on the inside. Now imagine how much water is given off in a real-sized greenhouse if you can, and add to that amount of water, the water given off from burning paraffin.
Finally, imagine what a perfectly-suited atmosphere it has become for moulds and fungi to thrive and multiply in; heat and moisture, mould and fungi heaven.
Greenhouse pests will also overwinter and have a head-start come spring when your greenhouse is filling up with seedlings and re-awakening plants, all at their most vulnerable to disease and pests.
Oh, and not to mention the cost. Did I mention the cost yet?
I have, after much trial-and-error, settled on a couple of 'golden rules' that, if kept to, ensure a healthy atmosphere for the plants that live in my greenhouse:
Plants can be wet and plants can be cold, but never the two together.
When outside temperatures approach 10deg C at night, I begin to start the drying-out process by watering plants less and less, and by the time temps are regularly under the 'magic temperature' of 10deg I'll have stopped watering almost all together.
A lot of the plants I think worth keeping over winter will respond by either dying back to a bulb/rhizome/whateveritscalled, will simply stop growing and wait for Spring, or will promptly set seed/fruit and die off..
... And I always allow that vital exchange of air between inside the greenhouse and outside.
Compost makes an excellent insulator against cold temperatures, when dry that is, and by using the larger-sized pots the amount of insulation is increased even further. I'll bet many would be surprised at what one can keep in an unheated greenhouse.
Hang on a sec, and I'll try to find the pics I took a few days ago..... Sorry they are only mobile-phone pics, I don't own a camera as such........ Oh, and forgive the mess of dead-looking leaves, the grapevine is busily shedding them like fury.
Cacti in particular, actually need a cold dry spell. It triggers their flowering mechanism, and my cacti flower regularly, year after year.
There's also succulents, Amaryllis, chillies, a 'Christmas cactus' back there on the shelf, a grape vine with it's roots outside but the rest in, Agaves, all kinds of stuff....
... and what simply won't overwinter, you can probably buy in spring for a whole lot less than your heating bill would cost you.
Hope this helps.
Nigel.