I have toyed for two years with the idea of artificial heat in the greenhouse (mine is 6'x6'). I reckon that electricity would be the safest and most economical, if I ever get 'round to it.
From March 'til May, I line the entire inside of the greenhouse with bubblewrap to retain what warmth there is. I also check the low/high temps in the open air as well as in the greenhouse this time of year until the weather is more reliable. Currently the lowest night temp is +8 degrees.
Any seeds that can't handle the fluctuating temperatures of the greenhouse germinate in the lounge!
It's what pasting tables are made for ;D
Art