Wendy Douthwaite also wrote under the pseudonym Elizabeth Wynne, under which name two books appeared as the Heronsway series.  From the badging as a series, I assume there were meant to be more books; but for whatever reason, it seems only two were ever published.  

Pony Quest (Heronsway 1)

Armada 1989, pb

Heronsway is a large equestrian centre.  Sandy rides there, and falls in love with chesnut Quest.  She

helps take care of her, and is asked to ride Quest eventing.

Rescue Team (Heronsway 2)

Armada 1989, pb

In the second book of the Heronsway series, Sue, the owner, has a terrible accident.  Sandy and

her friends fight to save the centre.