Fans who thought they'd have to wait until November to see David Tennant back on screen as  the Timelord in the latest of this year's four Dr Who specials have a surprise in store.  

He's due to put in a guest appearance in The Sarah Jane Adventures, a spin-off series to the cult sci-fi show, about a former Doctor's companion.   

His character joins investigative journalist Sarah Jane Smith – played by 61-year-old Elisabeth Sladen – in two episodes, to be seen in September.

The actress also played the character in episodes of Doctor Who opposite Jon Pertwee and Tom Baker between 1973 and 1976.  

She said: "When I heard the news that David was going to be joining us I was absolutely over the moon".  

Russell T Davies, the executive producer of both shows, described the part as "a full-on appearance for the Doctor as he and Sarah Jane face their biggest threat ever"