Where in south west are you exactly?
off the top of my head I can think of Therri Ikonen at Lime tree dressage, alice peternell at hinton (nr bath/bristol) and I think spencer wilton has moved to cirencester (correct me if I'm wrong)
another vote for Alice and Alex peternell. very professional yard but lovely relaxed atomosphere. One of the very few places I know where they personally are the only ones which will school your horse. You can find contact numbers if you search Hinton Dressage
Sam Twyman at Dovecote Stables is very good - he's Chard. Further north Adam Kemp and Matt Frost are good but they're in Gloucestershire and far more expensive.
The good thing about Sam is that he also has schoolmasters for you to have lessons on which I have found really help me. I can't recommend him enough really.
Two places, Sarah Cheetham at The Glebe nr Exeter, my daughter works for her and is super on tricky horses (trust me, I can guarantee that) or Sam Twyman at Dovecote Stables nr. Chard. both have websites and I can personally recommend both.