After being reminded about this a couple of weeks ago I decided to have a look at the stats back to 1806 and see what the results are.

If there is a run of cold days (3.3C or less) around mid-month the chance of an average to colder than average January and February is 94% and 74% respectively. The chance of an average to colder than average March is 96%. Also notable is that the chance of a severe spell in January (4C colder than normal) is 30%.

The full blog post can be found here. http://wp.me/p2VSmb-wO