Every link I can find (from Wikipedia to the BBC's own website) seems to indicate that Series 11 consisted of just two stories, with each of them being transmitted in two parts. (i.e. 4 programmes in total).
The two stories were "The Cave Woman" and "Fallen Angel".
As ever, Many Thanks Chris for your time and research...strange that they're stretching each part to a separate disk...hey ho!...and Thank You too Mel but I wouldn't have been able to read your link, so chris coming in like that saved a tinyurl being cast!...smile.
They probably don't need to 'stretch' the episodes to fill a disk. The Amazon website gives the total series running time as 500 minutes, so that's around 2 hours per programme.