This is the best price I've been able to find online. There's another posting on here showing a deal with Zavvi at £13.95 but when you go to the link it's actually £28.95.#
This is a continuation of Season 4. So if you have seen the 1-4 boxset which ends with the epoisode "Revelations", this is the remainder of season 4 that was released after the boxset.
I assume the first part of season 4 ended early because of the writer strike.