
Hi,
To my mind (and from personal experience) the best place to start is the Association of Pet Behavior Counsellors, accrediting only Trainers/Behaviorists who use only positive training methods.
This link is to the Essex area for APBC I would say it would be worth jotting down questions to ask any potential trainer/ behaviorist, you are going to employ them so you ought really to interview them! (This should weedle out any 'charlatans' but if you choose to use APBC this will have already been done ;) )
HTH.
(I'm just rushing out.. but will post more later, as you have my every sympathy having been there done that! I was incredibly fortunate to have found an APBC trainer straight off and now have a very mannerly wonderful dog due to positive training methods and the dedication of a wonderful trainer)