Hi Tricy,
I have watched "The Boy with the Two Heads" a few years ago, a very disappointing CFF movie from the seventies. The ones filmed after 1968 just are not as good as those from the 50's and 60's. I think it was also called "Chico" in Germany.
I have a few French Famous Five books and love the illustrations. I think Kirrin was called Kernac in the French books which sounded more like Brittany than Cornwall.
You are right, the summaries are very short in the CFTF Catalogue, probably because they have a lot of movies.
Hopefully you will find your missing movie. There are not too many treasure movies out there and even less with a wind mill included. I doubt it could be an American movie as there are almost no windmills left over here (one is on the isle of Nantucket and another one in the Hamptons on Long Island).
Please write again if you'll find out something. I'm very interested!
Best of luck,
C.