There is a veterinary academic also named Geoffrey Lapage writing at the same time as these books were published, so I wonder if they were one and the same. If so, the pony books provided some light relief from Veterinary Helmintholgy and Entommology. A Geoffrey Lapage, again I assume the same one, wrote The Ladybird Book of Bedtime Rhymes. His pony books were amongst the last illustrated by Anne Bullen. A Red Rosette and A Pony Every Time both feature Susan Cameron and her friends.
Finding the books: Both books are easy and cheap to find.