[Annieh78]

What temperature range do  you get over there?

Overnight in winter it gets down to about 2 degrees C; but most nights about 5 or 6 degrees.
In the days in winter the outside temp is about 11 degrees C on average; 15 if we are lucky!

At present the greenhouse is about 1 degree warmer than the outside temp.

At present there is only a single layer of polycarbonate as cladding but I have just bought some greenhouse film to try to insulate it (will peg it up in the inside somehow). Still, it is the icy concrete floor that is something I had not thought of at all in all my planning

[azubah]

That's better than we get here.. It usually gets down to -5 quite often here in winter and may not get above 0 some days. I would not put out delicate plants when it is under 10C.
You might have worse problems at the other end of the scale if it gets too hot in summer.