John Adams does clinics at Fenning Farm quite regularly, also, Ros who owns Fenning is extremely good! Have had her for a while now and can sing nothing but praises for what she's helped me to achieve!
Nick Turner, based at waresley park or Nat Dixon, or Val Gingell based in Horningsea very near Cambridge. All very good!
Would also recommend John Adams.
Richard Barton has a clinic each month at Ferry Farm Woodbatwick Norfolk.
He is brilliant & gets the best out of you. He can give you the confidence to push to that next level of your ability & he is not worried about getting on your horse to demonstrate either.
Janet Barton is a physio, treating both horse & rider which can help if there are physical problems making a horse go in a certain manner. She has appointments there as well.
Both have showjumped for Britain.
Highly recommended.