Found another interview with Mark Gatiss where he dishes on more of the stuff about this fab episode of Doctor Who. This one is with Entertainment Weekly.
“Ah, well, it’s the Doctor meets Robin Hood. That’s the most straightforward pitch there’s ever been, really! Steven (Moffat, Doctor Who executive producer) said to me, “Would you do Robin Hood?” And I thought, “Yes!” I’ve always been a huge fan of the Errol Flynn film (1938’s Michael Curtiz-directed The Adventures of Robin Hood), one of the greatest films I think ever made.”