The painting was owned by Sir Oliver Millar, who was the surveyor of the Queen's pictures from 1972 to 1988 and a leading scholar on the works of van Dyck.

BBC: Portrait returns to Palace 'home'

Sophie Maloney's two-year-old son Oliver, who has a rare form of cancer, spent two months at Nottingham's Queen's Medical Centre (QMC) in 2011.

BBC: Oliver, 2, and his mother Sophie Maloney