Doctor Who director Graeme Harper has reflected on filming one of the show's most memorable scenes – Rose Tyler and the Doctor's heartbreaking farewell in 2006 episode Doomsday.Harper, who helmed the season 2 finale, was speaking at BFI Southbank's Doctor Who: 20 Years On – The Tenth Doctor event following a screening of the episode, which famously saw Rose (Billie Piper) trapped in a parallel universe and separated from David Tennant's Tenth Do…