Which is the cheapest way to heat a greenhouse

If you want to keep it just frost-free, then probably insulating it with bubble-wrap would do it, plus fleece over any vulnerable plants on colder nights - however, an electric fan heater on a thermostat is your best bet. Paraffin and gas heaters bring extra problems with them - condensation, fumes and the danger of running out of the fuel at the wrong time!

Thanks Spritzhenry thats just the answer I was looking for
garden2010

You're very welcome. :-)

I use Bubblewrap and up to three paraffin heaters in my 6' x 8' greenhouse as I grow chillies and have no electric.
Paraffin is bought from the local allotment association (3 quid per gall.) as its a lot cheaper than buying from a garden centre (5 quid +).