Prices vary - there is no set price for Cocker puppies and some breeders just charge what they think would-be owners would be willing to pay. Factors like KC registration, good health test results for both parents (DNA tests, eye test, hip score), proven ability of the parents (working or showing) can all influence price. As £800 is at the top of the price range (working type litters are often much less than this), I'd probably expect all the above factors to apply, particularly the health testing but really it's your decision. Can't the stud dog owner help you with advice on this, especially if they are local (or your breeding mentor if you have one)?