rideaway near york? or beavers in beckwithshaw? or fox saddlers in wetherby? I think they will all come out. I
have had Mrs Fraser from beavers fit one before but not recently, and I don't know if any of them are technically qualified, but could be worth looking them up. think I got the back man check that mine fits (had laready bought it and i thought it fitted). I think they will part ex if you buy a new saddle, but not a second hand one - but again worth checking as not sure if all of them have the same policy.
Keith at Fox Saddlers is pretty good - I think he may be qualified and he will come out (if you're near enough). We usually just borrow a few at a time to try from them and they will part exchange.
This is the web site for Hey farm Saddlery in Huddersfield they have a good selection of new and used saddles and will do part exchange as well they will come out or you can take the horse to them.