Yeah, definetly had the city agree to it, no way in hell to sneak a statue like that without the cops getting involved.
Westminster City Council has told the BBC it did not grant permission, as it was not given advance warning that Banksy's team was planning this installation.
Council permits are usually quite public (in my country). Sneaking it in becomes part of the artwork.
(Though it's not in /the/ City of London. That wouldn't happen in a million years! City of Westminster is way more culturally flexible)
The City is dead at night. If an artist wants to put art there, they'd just as somebody else said, dress up like they are workmen and be fine.