In the absence of electricity you won't [AKAIK] be able to use a decent thermostat - so you will have to light on the nights you think it may be cold, and extinguish the following morning.
Bottle Gas will have a crude bi-metalic strip type thermostat, so will only use gas for a pilot light when temperature is satisfied (but the inefficiencies of the thermostat will mean that the greenhouse over- (or, perish the thought!) under-heats by maybe as much as 10C before the thermostat cuts out.
Paraffin has no thermostat, plus also the risk of the wick burning unevenly, and sooting-up the whole greenhouse, or going out.
Both generate considerable condensation [compared to electric], and both also rely on convection to get heat into the far flung corners of the greenhouse - an electric fan is obviously better at that. One counteracting approach is to put the heater under metal staging, which then heats the staging and thus the soil in the pots stood on it, which the plants appreciate (and often means they will tolerate lower temperatures at their leaves).
Either way, insulation is the cheapest form of "heating", so well worthwhile attending to that first - e.g. lining the structure with bubblewrap. Make sure that all air gaps are sealed with tape - you need to reduce all draughts as well as "double glazing" the structure. You could also look at keeping wind off the structure - a panel-fence, or hedge, around the greenhouse (but not close enough to shade it) will keep chilling wind off it.
Another trick is to erect a small polytunnel (like a very large cloche) inside the greenhouse, and only heat inside the tunnel. The greenhouse will provide the first line defence of keeping cold wind etc. off, and inside the tunnel will be a much smaller volume of air to heat.
But in a cold snap temperatures will fall, no matter how good the insulation is, and once the temperature falls you will be relying on the heating to stop the temperature falling further. To achieve that you need X-killowatts, so T-lights and so on won't be enough ... if we have a mild winter then very low numbers of Watts will be sufficient, in a perishinlg cold snap you'll need all the Killowatts you can throw at the problem.
My solution to the problem is to have substantial growing lights, in the house, and overwinter my plants there. I have sufficient light "power" that they continue to grow, rather than just ticking over, so I don't have to worry about moulds and rot and so on, let alone cold that could kill the plants.