t-bag - You are absolutely correct.
"Am I right in thinking therefore that the category rating of each airfield is a matter for decision by the operating airline, not the CAA? " Yes
The Cat C designation is made by the operating airline. For example Mykonos is Cat C with some and not with others.
The only Cat C charter summer destinations that are common to most operators appear to be Samos, Funchal and Skiathos.