I ride at Dulwich riding school. Been trudging round a barn now for about 20 years (not continuously, obviously). My first time there I just told them my experience, and I had a group lesson as an assessment, then they told me which lesson I should go in. It's ?22/hr, and the instructor is one of the best I've met. I'd definitely recommend it. And they supply hats and boots if you don't have your own. PS thanks for the website link, I'm off to be nosey!