Looking at the old site you're quite correct, I'd forgotten all about it! However, I do state incorrectly there that the map is from 1856 when it is actually from 1819.
Of the post mill, the Mill's Archive says that "It is not clear why this has been filed with the Berkshire material" and states that it is in Kent. I can't find a Cox Heath in Berkshire but there is a Cox Green so perhaps this is where there's been some confusion.