Mary de la Mahotière wrote just the one pony book: it is a sequel to The Newspaper Children, which, as far as I know, is not a pony book. The book is illustrated by Marcia Lane Foster, who was particularly notable for the way she designed her jackets so that the spine was an integral part of the whole.
Mary de la Mahotière was born and educated in Devon, and then studied modern languages at Bedford College, London, She worked for the BBC and the Political Warfare Executive, during which time she met and married her husband: then in the Rifle Brigade and Intelligence Corps, but who later became a journalist. Mary de la Mahotière’s books were set in the Devon she knew and loved from childhood, but she and her family had to make do with occasional escapes to the county from Hampstead.
Finding the book
Generally easy to find and reasonably priced.