William Willoughby was the stunt double of Mitchell in the Episode Another Fine Mess. He replaced Aidan Turner when Mitchell and Bernie got hit by a car. Will Willoughby has also appeared in many different films and TV series. He was Benedict Cumberbatch's stunt double in Sherlock and appeared in Doctor Who, Merlin, The Bill, Torchwood, Game of Thrones etc.
Here is a video interview (1.40 min) of the Will Willoughby: