Darren, many years ago I went to stay in a friends house which was the old railway station at Penrith - yep, it was colder inside than outside even though the walls were around 4" thick! (He wouldn't turn the heating on in the middle of winter!) We've never let him forget it!
I currently have one of those plastic greenhouse covers set up in the normal greenhouse to provide protection, as well as a couple of those baby plastic greenhouses - seems to be working OK so far - having said that with the way the temperature is going to drop this week, shall wait and see. (Shan't be going outside to check - its too cold!)
DDL