With those temperatures I think your limiting factor is going to be sunlight, not temperature.
Assuming you are not trying to grow tomatoes in January I'd think a single layer would be quite sufficient. Here in the US the double layer of plastic (not the bubble stuff) is common and is inflated to insulate. My winter temps are about the same as you or a bit colder and I only have a single layer of plastic. There's maybe 1500 litres of water as thermal mass.
Actually come to think of it I do start some tomatoes in January.
posted 2 years ago
Your comment made me think of adding a reflective sheet to the north side of the tunnel. So I might give that a go..
please buy my thing and then I'll have more money: