With hypothermia, one of the big problems is that once you are past a certain point you don't know you are suffering and therefore don't take action.

I suffered on the carneddau once in a blizzard and hadn't even done my coat up. Fortunately a mate realised what was happening and physically zipped me up. We then dropped height rapidly and within 10 minutes I was OK.

Also had a party member who was very cold, and whilst normally very chatty, went quiet. We had to take her jacket off and then dress her in spare clothing that she was carrying. So watch out for out of character behaviour.