Dark Knight DVD is on offer instore at W H Smith for £6.99 when you spend £20 on books, Xmas cards or stationery.
It's the 2 disc special edition. As far as I know it's nationwide and instore only.
I've picked up a copy myself so it definately is on offer.
Will post more details when I have time.
(Been buying from tips here for a while so thought I better return a few favours. First post so go easy etc)