Okay: (1) The acting *is* wank (except for Dennis Price); (2) the script is hilarious (One classic exchange: "If Penny didn't kill those kids, then who did?" "Someone else, obviously."); and (3) the "twist" ending is absurd (although it predates by one year the strangely similar finale in _Don't Look Now_).
But: This movie is lots of fun. AND it can still generate a couple of screams!! Imagine a live-action, British version of Scooby Doo for adults. This movie is the product of the same time and mindset as the cartoon. It has everything the horror movie fanatic could want: gory murders (even the old severed head rolling down the stairs), secret tunnels, buried treasure, a hint of evil spirits, madness, a monster, sexual intrigue, gratuitous nudity, and lots of creepy atmosphere. That island is the most foreboding piece of land I've ever seen.
Check it out.