A lot of my friends have used Martin & Mark Edwards (Mark does most of the ridden work these days) in Llandow and have very nicely trained horses as a result. Mark's a showjumper but started and trained lots of horses as all rounders. They have good facilities - indoor school and lots of their own land for hacking and are nice friendly people.
Serena James is also good. Her facilities aren't quite so good (but plenty good enough!) hence lower price, but that tends to mean any horse she starts will have done a decent amount of hacking by the time you get them back. She's a really nice girl too.