The Witches Mill was a wonderful place of adventure and wonder (and fear!) for young impressionable minds like mine in the early 60s - I was adicted to reading Dennis Wheatley, Edgar Allen Poe and the like and watching Hammer films for quite a few years as a result of my visits! As a tourist attraction it had a certain weird charm - I remember the cafe (Witches Kitchen) and a menagerie and the collection of occult objects and the like. Having seen the film of The Devil Rides Out, it took extremely courage or bravado to enter the room with the pentacle laid out on the floor in the upper floor of the Mill.
Manx Notebook has some information on the Mill's history and I tend to agree with Frances's assessment that the witchcraft claims were all nonsense fabricated to attract visitors. Gardner's book on witchcraft was offered on e-Bay recently and sold for a ridiculous price, so there is still some interest out there. The occult/withcraft collection was sold to Ripley's Believe it or Not museums in Great Yarmouth and USA - so it has not gone completely, just mutated and migrated!