I think there is probably a good lift somewhere in the building (old warehouse). There is a whole other side to the set as well so probably another entrance route there.
I assume a stagehand takes it some up service elevator. The stairs are just there for show.
That's what I was thinking, but it must be a huge elevator, something that you don't often see inside. Whatever they do, it must disrupt the goblin who lives downstairs...
Peter and Duncan were also on The Chris Moyles Show yesterday morning, I was half asleep but I'm sure I heard Peter say that todays episode features the next big thing since Levi Roots' Reggae Reggae Sauce.