The Ponies Loved It Too was written by both Paddy Miles and Rosemary Griffin. Paddy Miles doesn’t seem to have written anything else that I can find: Cheers for the Ponies and The Ponies Camped Too seem to have been written by Rosemary Griffin alone, though the dedication of Cheers for the Ponies is 'To Paddy, with many thanks for making me write at all.' Presumably this Paddy is the same one as the joint author of the first book.
Finding the books
All the titles are reasonably easy to find, and generally not too expensive.
Links and sources
The British Library
Many thanks to Amanda Dolby and Susan Bourgeau for their help with the pictures.