Believe it or not, properly designed and fitted thin thermal under layers will do a much better job of keeping you warm than a bulky jumper or hoodie. Here's a run down of some of the top thermals available, compiled by MCN's sister title RiDE magazine.
Try to buy thermals that are a good tight fit as this will help keep you snug by trapping warm air against your skin. Also upper thermals with a turtle-neck are good as it helps to form a seal against the draft.
Good thermal under layers should be breathable so that they don't retain sweat. Check the seams and material used, make sure it isn't scratchy as this could get uncomfortable on a long ride.
Here's RiDE magazine's run down of the top 5 winter thermals available: