Currently £8.99, nearly the cheapest it's been. Cheapest according to IsThereAnyDeal.com was £8.74 on Amazon UK a couple of months ago.
It's currently listed at £39.99 RPP on Steam, so even if it hits a 75% discount in a sale (unlikely, we'll probably see 66% first) it'll still be more expensive than Amazon.
Outside of a great deal + voucher from GreenManGaming, or a copy from Nuuvem (and all the regional fun that brings), this is likely to be the best price on it for a while.
The game comes highly recommended and well-reviewed.