certainly it offers some protection to plants but if a big freeze occurs then pots do freeze in there. Very good for keeping winter wet of plants though. It is very useful for hardening off small plants in spring.

You might try lining it with bubble wrap as you can do in greenhouses, which could give you another degree or two of leeway. And you can cover it with an old blanket or something if a heavy frost is forecast. But anything precious would be better inside.

Thankyou Sbg and Stera. I'll take note of your comments and advice. It's my first cold frame and I've already been doubting its ability to keep plants frost free. So I'll definitely try the bubble wrap trick and move some pots to a warmer place. It's my cuttings I'm most concerned about. I don't know whether I'm imagining it but it seems much colder here in Devon than usual for the time of year.