Antony Hopkins (b.1921) is best known as a composer and writer on music. Lucy and Peterkin seems something of a departure, so I’m hoping the British Museum is correct in its cataloguing and that the composer and author are one and the same!
Finding the book Reasonably easy to find.
Links and sources The British Library
Bibliography (pony books only)
Lucy and Peterkin
Leslie Frewin, London, 1968, 81 pp, illus Graham Mansford