Guest Stars Susan Twist as Mrs Merridew Bernard Cribbins as Wilfred Mott Nathaniel Curtis as Isaac Newton Daniel Tuite as The Doctor Acting Double Helen Cripps as Donna Acting Double Ophir Baray as The Doctor Beast Double Tommaso Di Vincenzo as The Doctor Contortionist Double Uncredited David Tennant as The Doctor No-Thing Double Catherine Tate as Donna No-Thing Double