Thanks for the replies.

Unfortunately, I don't yet have an electricity supply to my greenhouse, which is a job planned for next year to coincide with electric to power my pond pump. Both are a decent distance from the house so a temp hookup isnt really practical - which is why I bought the parrafin heater.
I think if I were to change and buy a gas burner the additional cost would outweight the saving in fuel initially - so what I need to find is a cheaper or more efficient way of running the machine I have bought. It is a big heater but my greenhouse is 10x 12.... It says in the manual that the burn time is 18 hours. We tried to have it as low as possible on Fri night, but it was still almost empty by morning, though I guess as it is new the wick will take a bit up before it gets started.
I have researched using kerosene as we have oil CH but decided against it for the safety of my plants (and me!).
I was reading of a place near Manchester that did a self fill parrafin supply into containers , and hoping there might be something like that in our area?