Well thanks for all the replies people, very interesting.
I don't get cold on the bike, far from it, the lowest I have ever done the journey in is -11, but on the very cold nights when I get in and get changed, I get cold quickly. When I am on the bike even though it below freezing I will still sweat. So I don't think the cold is the problem. I get the shakes when I am sat down in front of the telly !!
I have the spiked tyres, Continental they are excellent, but I only tend to fit them when there is ice for a few days as taking them on and off is a right faff in the evenings, I should get another set of wheels, but that will have to wait. So there's no problem getting there and back to work, but....
I have been cycling to work now for 3 and a half years, coming up to 24,000 miles, its a long time on the bike, so I had toyed with the idea of getting a motorbike rather than take the car on the odd occasion. With the motorbike I can still park at work, but with the car that's a different story, there's not enough parking and you only get a space once every 6 weeks. In Cambridge parking at work is a disaster, so that's where the motorbike idea came in really. But its all just thoughts and so I was interested in what others thought.
But I certainly thank you all for the replies, its good to get the word from the horses mouth so to speak.