Who is behind that anonymous major art purchase at last month's Sotheby's auction?

At the recent Sotheby's Contemporary Art evening sale in London, a Basquiat sold for £28M to an anonymous telephone bidder, dramatically above estimate. The art world is buzzing about who it was — speculation ranges from a Gulf sovereign wealth fund to a specific tech billionaire who has been building a collection quietly. Anyone with inside knowledge of the auction house or the buyer?