I have grown Maurelii,Montbeliardii and plain Ventricosum all out side unprotected and all three survived. They were very late to re start in the summer and never made good growth for the rest of the season. The lowest temps were-8
If protected you should be fine. Its a bit late now as we have had some very cold nights and damage may have been done. Fleece would be a good option like Kev said. Failing that a tempory rain shelter over it and then a fleece wrap round to keep out the wet and the cold.
Here is a link with a little info on what has come through for others in the UK
Hardy Tropicals - View topic - cold hardy nanas
If you click on search you will find other threads with pictures ( mine inc)