Thank you SmokingCrow. The tunnel took me 2 weeks to build. With winter coming I am looking for a suitable and economical heat source to install. I will probably end up going the solar panel route. Haven't made up my mind. Maybe you have some ideas.
There are loads of diy solar 'furnaces' online and it seems one of the better/easier designs is to make a box, like a drawer. The bottom and sides of the drawer are wood but insulated with polystyrene or similar and the entire inside is painted matt black. Make a narrow frame that fits inside the drawer and attach some fine metal netting (screen door/mozi netting) onto the frame and paint it black. Fix the frame so it sits parallel bottom of the drawer dividing the depth in half. Then glaze the drawer with glass, double glazing if someone is chucking out a window/door. If you've got a piece of glass, make the system fit that glass, rather than cut the glass and lose some collecting area. Drill a large hole(s) in one of the narrow sides, This will be the 'top' of the solar collector. The hot air will come out of the hole(s) and a pipe(s) is fed into your poly tunnel. On the other end, intake holes are drilled, and the ambient cool air is drawn in, and it is heated, hot air rises and it naturally flows into the poly tunnel.
There are similar designs using aluminium cans, drilled, stacked glued and painted black, but it's a hellova schlep compared to using a staple gun and some fine wire mesh and on test I saw, the differences were minimal. (found it) Of course the drawer would have to be aligned to face north at an angle but you can make adjustments and check the output temperatures.
An easy way to keep the heat at night is to use drums filled with water, they get hot during the day and act like a heater when the sun goes down - google thermal mass, there may be better options but water is quick and easy. Another thing is that you can also use it for watering plants so they don't have temperature shock when cold water comes out of the hosepipe. You can fill the tanks with water in the morning and it's going to be nice and warm at night. I'm going to use some old jerry cans, I pulled out of a skip. I've got about 10 that I'll paint black to help absorb the heat.
