Hodder & Stoughton, London, 1958. Cover art uncredited, but I think it is by Caney, who also illustrated Hodder’s Jill books
The Hardcastle children are orphans, living with their grandmother, Mrs Aston-Pringle. She is smart and wealthy, and spends much of her life competing with her “greatest friend”, Lady Pinkney. Shamus and Tim O’Brien are Irish, scruffy, live in a crumbling though vast house, and earn their living through horse dealing. Mrs Aston Pringle is determined her grandchildren will beat the Pinkneys at showing, marry well, and that no one will have anything to do with the O’Briens. The children, however, have other ideas.
Many thanks to Amanda Dolby for the picture.