TDS Saddlers. I know he isn't everyone's cup of tea, but I found him very good.
He let me play to my hearts content with all the yummy expensive saddles! I got a good deal and a yummy new saddle that fits like a dream.
Failing that, Elly from the Bespoke Saddle company, she is a good fitter and she makes her own saddles if your horse is tricky to fit. She has a website if you google it. If not, PM me and I'll give you her number.