Check your school holidays! In my part of Wales terms ends on 23 December (it's the week before in much of England) and terms starts back on 9 Jan (really late!).
Center Parcs obviously don't realise this as I've just booked a 3-bed Executive Villa at Longleat for £419 for a mid-week break for 2 - 5 Jan 2012.
For comparison, it costs £859 for w/c 19 December 2011and £1099 for w/c 2 April 2012.
This is unbelievabley good price for Center Parcs during the school holidays!