Have you got any control over the heat - I mean, to regulate the temperature in there? I think it might cause the same problem as paraffin, i.e. condensation, with the risk of botrytis.

Is there any chance of running a cable out to take electricity out there for a thermostatically contolled fan heater? You have to have any installation checked by a qualified electrician these days, but it would be worth it for the future - you might want a propagator as well - or even lighting!

Spritzhenry is quite right, and I would add that using something not designed for greenhouse use would mean it is far more likely to produce noxious fumes which would affect the plant. A patio heater probably has a very basic burner and wouldn't be adjusted for minimum emissions.
It would also work out MUCH more expensive.
The advantage of electric heaters are that you can set many of them to a 'frost' setting and they therefore spend a lot of their time just ticking over. It's convenient and very cheap.
Installing electricity is not that difficult provided that the wiring is contained in the thick plastic tubing you can get for the purposed in DIY stores and that you use the proper 'outside' type of sockets and so on.
I always switch the power off at the source before watering or doing anything else at all 'risky' as a double safety check (having experienced some nasty electric shocks in my youth with dry feet and inside.. with wet feet in a greenhouse they'd probably be lethal!)

I have been doing some experimenting with this heater it has got 3 temp setting and is keeping the greenhouse at around 63 to 66 degrees constantly with vent open on the lowest setting both day and night. So will that be OK? Where the greenhouse is positioned it gets a lot of sun and during sunny days and keeps them sort of temperatures without heaters

Keeping that high a temperature must be costing you a fortune! I hope the plants are very special ones...I'd still be unhappy at using a heater not designed for greenhouse use, though.