Heating - without electric

I think there are two choices:

0) Electric.  Fully thermostatic, and as such I think the cheapest for Spring frost protection. Only on when needed, and apart from any worry of power cuts?? no need to visit greenhouse of an evening to "light it" etc.  Cost of fitting an armoured cable to the greenhouse, and a mini distribution board and an earthing rod is ... significant and, of course, a non-starter for an allotment or site remote from house.

1) Bottle gas. Runs on a pilot light, and a crude thermostat, so not too wasteful.  produces some condensation, thermostat is crude so Max/Min temperature is a bit of a guess. If you already have a bottle, for BBQ/similar then only cost of fuel, if not there is cost of the "rental" of the bottle.  Need two bottles to be able to switch over when the primary becomes empty.  Dunno if there is much chance of the pilot light going out?? Risk of the Bottle being stolen on an allotment?

2) Paraffin. Lots of condensation, risk of it sooting everything up if the wick burns unevenly, need to light it manually on cold nights and extinguish in the morning, so it will be burning all that time, regardless of whether the temperature is/remains cold or not, so expensive on fuel.

Either Gas or Paraffin can be put under metal staging and will heat the staging, and thereby the roots of the plants, which will enable them to endure colder air temperatures.

You might get away (for a small enclosure, within a greenhouse) with Tea-lights - an upturned clay flowerpot over the top will help to spread the heat.  Has the drawback of having to light them in the evening when cold temperatures are expected (although no need to extinguish them in the morning I suppose?!!)

I tried them but I just could not cut the pots in half like the diagram 

tea lights only burn for a short time and would never last overnight

You can buy eight hour tea lights

ebay £7.99 50 Pack - 8 Hour Burn Long Burning Wax Tealights Night Light Candles Christmas candle

I've seen various calculations for the wattage of Tea-lights, but they range from 40W to 80W.  It isn't much, although in a small tunnel / 4-shelf blowaway, inside a greenhouse, it isn't a lot of space to heat so may be viable.  But important not to assume that a Tea-light is enough to heat a greenhouse that would ordinarily need a 2kW fan heater - that fan heater has the equivalent output of 50 Tea-lights

Some insulation over the heat source and plants would reduce heat loss ... an old duvet over the top perhaps?

Fire risk too of course ....

Or just wait for the weather to improve 

It's all related to the specific heat of the storage material, and how long it takes to release the stored energy.
Here are a few specific heat examples of some common materials  and the energy to heat them  in (J/kgoC)

Water 4189 (J/kgoC)
Air 1000 (J/kgoC)
Sandy clay soil 1380 (J/kgoC)
Dry soil 800 (J/kgoC)
Wet soil 1480 (J/kgoC)
Terracotta 1000 (J/kgoC)
Remember that the weight of air is 1.23kg M3. So lets say your terracotta flower pots weigh 4kg and you heated them up to 90 c, the same energy would heat 1ltr or water to 90c  in the same time. As the specific capacity of air and terracotta are about the same 1000 (J/kgoC)  they cool down 4 times faster. So you need to ask yourself could a 1 ltr container of water in the centre of your greenhouse with a constant  temperature of  90c heat your greenhouse over cold frosty nights. remember we have not allowed for any heat loss through the thin glass.  so please don't think a few flower pots and and a couple of night lights will keep your plants nice and warm  there are lots of other better materials in the greenhouse that are better for storing heat.