Thanks for all your replies. There's certainly a few things to try out there.
I can just imagine the situation.... "Honey, would you like to come camping with me?", "Why do you suddenly want me to come camping in the middle of winter?" the wife replies, " So I can use you as a feet warmer!!"
or
"Honey, I've changed my mind, I would love to have kids now", "why the change of mind?", "so I can shove them down the bottom of my sleeping bag to keep my feet warm!"
The down socks sound like a good idea as does heating the rocks/platypus/nato bottle. Wearing a hat doesn't seem to help but it might well be a problem with my sleeping bag. So many things to try...
Thanks again.