Was a series of roughly 6 books, think the main character was named Alex. She had a (dun) horse that I'm fairly certain was named Barney. In book one the horse was rather naughty, and went charging through an old man's garden. Said old man (Eric) happened to be a retired professional trainer, and took Alex and the horse under his wing and trained them both for competing in 3 day eventing. Main character had a boyfriend named Ash (I think). In one of the books a former groom of Barney's turned up, and Alex ran away with her horse to avoid him being taken off her as there was some sort of scam re. a claim that Barney had been sold illegally. I think that the series may have been called 'riders' (definitely not Jilly Cooper), but haven't had any luck finding it on Google.