Did taxpayers pay for Ali C's Hutton press conference?

Something I don't think many people noticed occurred in yesterday's Independent. Alastair Campbell's "presidential appearance at the foot of a grand staircase" on Huttonday was described as an "event arranged by Downing Street". Now this is interesting. I (and everyone else, I'm sure) suspected as much. What is Private Citizen Campbell doing as the Prime Minister's Alternative Official Spokesman (acting, unpaid)? What interested me was how much it costs to hire the Reception Hall at the foot of that grand staircase in 11 Carlton House Terrace, and who paid. Did we foot the bill for Ali's egofest? I've enquired with the Foreign Press Association as to exactly how much that would have come to. We shall see.