It has started (Dec 21st/22nd), but is slow at first and for some reason the evenings draw out quicker than the mornings.
Here in Bristol the sun sets about 4.20pm instead of 4pm but the sunrise is not much different at 8.16.
By the first week in Feb you'll be able to work until 5pm.
When the clocks "go forward" at the end of March you'll really notice the difference in the evenings.