http://www.ebuyer.com/570064-zoostorm-haswell-desktop-pc-7873-0494
They also do a version with an AMD 4.1GHz processor that only costs £269.99 after cashback, but it doesn't include any operating system. If I were building a Linux box or a machine primarily for games I'd be very tempted.
Now I need to get some CAT6 cable to run from the Gigabit socket on my router to the PC...