Mitch was a friend of Anji Kapoor.
Before she started travelling with the Eighth Doctor, he had gotten a job in Edinburgh and told Anji about it. When she thought she had returned to her Earth, she tried to visit Mitch. However, when she could not even find the company he worked for at the address he had told her, she started to realise she was on an alternate Earth instead. (PROSE: The Domino Effect)