 Originally , the producers considered having Michael Obiora play both the young and old version of Billy Shipton . However , it was decided that Obiora in makeup would look too fake , and so Louis Mahoney was cast to play the older version . Initially , Obiora played the role with a London accent ; Mahoney , however , has a thick accent . Obiora had to re @-@ dub his lines to match . Billy mentions that the windows of the TARDIS are the wrong size for a real police box . In 2004 , when the first photographs of the new series ' TARDIS prop were revealed , there was a vigorous discussion of the box 's dimensions on the Outpost Gallifrey Doctor Who discussion forum , in which some fans complained that the prop 's windows were too big . Moffat has confirmed that this line is an in @-@ joke aimed at the Outpost Gallifrey forum . 
