I know John personally. I started taking occasional lessons with him when I was 12 and still do (again, occasionally.). He is really nice and personable and has done ALOT in his eventing career (he's gone to Rolex, been long listed and short listed for two separate olympics). He is really good with horses that can be pushed and can do quite alot for you in that department. I spent a week with him when I was 13 at his place (REALLY NICE PLACE!!!!!) and found it very educational and my horse and I got a lot out of it.
He is pretty good with green horses but in my experience (not everyone's just mine) sometimes he can be a bit pushy about what he wants you to do. But if you really are firm and say you don't feel comfortable doing something, he is quite good at backing off.
I definitely recommend you take some lessons with him and see what you think. I haven't had someone yet come away unhappy from lessons. He is a good coach at events, too.
His website is www.windermerestables.com
What was the name of the person that recommended him? Just curious because I know of almost everyone in the area that knows him. PM me if you feel comfortable.