Graham has himself confirmed this on Twitter:
Seems cat's out - I have written eps 5&6 of the new WHO. I am thrilled with them. Filming starts on Monday. Needless to say, I can't reveal anything but it was a joy working with Steven, Piers and Beth - not to mention the superb cast.
The episode titles are: The Rebel Flesh and Gangers
The Life on Mars/Ashes to Ashes creator previously wrote Fear Her for David Tennant's first series.