BBC2
It is on BBC2 where both series of The Office and Extras were shown. Although the BBC offered Ricky Gervais and Steve Merchant the chance for Extras to be shown on BBC1, they alledgedly turned it down.
BBC2 typically recieves less viewers than BBC1, and so Extra would have had greater viewing figures if shown on BBC1. It is rumoured that the Christmas Specials of Extras will be shown on BBC1.