He was recommended to me recently by someone who works for my dressage trainer (and who herself has evented to advanced) - I don't know what he's like for selling a horse but she suggested he'd be good to go to in order to buy from. I have his number on my pinboard waiting for the moment when I am ready to start looking....
I can't say that I know Mark personally or businesswise, but have photographed him on several occasions this year. He has this habit of asking the arena party to roll the top pole on a fence that he has noticed when entering the ring for his SJ round that he feels requires adjustment.
Can't honestly say if this is simply very close attention to detail or just a mannerism he has developed, the way we all develop habits. I suspect a bit of both.