Overmitts, especially if it's damp, are great. If you can't get any double up on gloves. (I use little light gloves for warmth and dexterity and the mitts to keep them dry and maintain warmth).
A decent warm jacket (named fabric - goretex, eVent and the like) has been a worthwhile investment over the winter and it got some use again the other day (in the centre of Sheffield on the commute!).
Survival bag incase it all goes Pete Tonge!

as ever exercise sound mountain judgement and if you start to feel cold get off the hill as safely as possible.