Late addition here.
Too cold in winter, too hot in summer.
I'm embarking on a project in the next few months, I have used a gas heater (very efficient in the very cold), I have the luxury of a power supply that has an oil filled radiator on frost stat in case the gas runs out. My idea is a bit different and there are pages here and there on the net that describe this more than I will here.
Step 1: (the biggest job)
Lift greenhouse and dig 1metre deep trench through centre where the greenhouse stands. (although I'll be making the trench almost as wide as the greenhouse). Line the base with plastic guttering tube with holes in the tube, have 90degrees at each end to route the tube above ground level. Then fill the hole with pebble (pea gravel), I'm researching the best here. It seems the volcanic type is the best. It needs to be full of holes to be most efficient. Refit the greenhouse (probably buy more glass as well).
On one side I will still have a border, however, I'm thinking of hydroponics this year to grow my peppers and chilli and maybe tomato. The tubes from underground need to be joined to either end of the routing underground, one goes up in one corner just above the ground and then along the length of the greenhouse on one side (with holes in it), and the other the opposite corner all the way to the very top of the greenhouse.
I'm mad on solar power and have several solar panels dotted around that power lighting in my garden at night, All the batteries for these panels are in the greenhouse so the power is there for this step. I'll be fitting a fan, probably a 120mm pc fan that can run on 12v on each end of the routing at the top of each 'chimney'. One push and one pull. I have done some maths on this (I'm a mechanical design engineer by trade), and this should work easily.
According to my calculations, during the summer the auto vents open fully to let all the heat out during the day. This system should drag hot air from the very top, through the cool pebbles underground and blow out cooler air into the greenhouse, at the same time, they will warm the pebbles. During the night, the same warmed pebbles will keep the greenhouse warmer with the same flow. Winter would have a similar effect, however, not as much. I have seen my vent open on a very cold day shortly after the sun comes out and generally the greenhouse is warm enough on a bright day (unless it's covered in snow). This system should ease the yearly cost (£80 LPG for the gas alone).
I guess it's not an ideal solution for winter heating, however, for the summer it's ideal. I'm expecting good results in spring and starting things early. I have trialled a few things and was surprised on the poor results of bubble wrapping the inside of my greenhouse, I tried the black water containers and that gave some results. I have trialled a small scaled model with the brown pea gravel in a large bucket and it gave fairly good results. I also have a problem of water, when it rains heavy the pea gravel will fill with water and that would not transmit heat well, so maybe I will have to come up with a way of moving air around just above the flood line and drill some drainage holes. I would also think that the movement of air would be good for the greenhouse.
Any comments appreciated.