I emailed English Heritage, and they had this to say....
"Good day Maelwen,
The Winter Solstice celebration is usually much lower in key than the Summer celebration. Usually there is a Managed Open Access between about 7.30am until 9.00am, weather permitting. This year it will be on Monday 21st December 2009. There are upwards of 100 people for this and no booking is required. No parking is offered though. Please see www.english-heritage.org.uk/stonehenge or possibly www.english-heritage.org.uk/wintersolstice in due course for further information.
Helen"
I'm making the trip that morning from Bristol with my father. It will be my first visit to Stonehenge, I'm very excited!
BB,
Maelwen
xx