I don't know anything about the sales side, but I went for a lovely ride on Kingley Vale many moons ago (20 years +) and the horses were exactly as we were advised. I would assume the fact they have been trading for so long means they must be doing something right.
Sorry, not much help with your potential purchase!
As a local I've never heard anything bad, but on other hand, I don't thin I've ever met anyone who's bought from them either. It's been going years though and selling for years so assume that's a good sign.
Am I the only person who read Willowbrook Riding School in thread title and immediately pictured Thellwell's fabulous Willowbrook Riding School ponies and riders?