Even before she became Prime Minister, Theresa May pledged a new government department that would oversee Britain's departure from the European. Not just that, but the department would be run by a leading Brexiteer. How much influence did David Davis - the first Brexit secretary - have? Who was really leading negotiations? And was it all simply a political manoeuvre on the Prime Minister's part? Mark Mardell delves into the corridors of Whitehall and the new Department for Exiting the European Union, to find out what was really going on behind the doors of No 10 and No 9 Downing Street. Email: pm@bbc.co.uk Twitter: @BBCPM