Cold feet, wear a hat. Mittens will help as well to keep other extremities warm as well.
Goretex Socks and some MoD Issue Extreme Cold Weather Socks also.
http://www.strikeforcesupplies.co.uk/index.php?method=stock&id=20152&from=79Chemical hand warmers have always struck me as being too heavy to carry around for what they do compared to the weight of an aspirin.