A logjam of giant names in the arts comes together in one small canvas to be auctioned next month: Lucian Freud, painted by Francis Bacon, and owned by the late Roald Dahl.

Dahl died in 1990, Bacon in 1992, and Freud in 2011. Although they later fell out, the young Bacon and Freud were close friends, who painted one another's portraits – and Dahl was a great admirer and friend of Bacon's.

The renowned children's author bought this Study for Head of Lucian Freud in 1967, the year it was painted, for £2,850 with the proceeds from one of his most famous books, Charlie and the Chocolate Factory. It will be sold next month at Christie's estimated at up to £12m.