I remember watching this on VHS years ago, along with other British horror movies such as Horror Hospital, and catching it on its occasional TV showing. Right from the opening shot of a creepy model of the island and its lighthouse, I was hooked.
The story starts with the discovery of the bodies of murdered kids on the island, told by a survivor. This provides a number of grisly scenes. The film is almost like an early 'stalk and slash' movie. Once this back story has been dispensed with, we're with a new bunch of characters who are heading to the island looking for treasure... Except the killer is still there. As with later 'stalk and slash' movies, there's an emphasis on sex before death... But this predates those by some years.
This is not a perfect film, though. The story sometimes drags and the sexual shenanigans get in the way of the story, but it's well worth watching if you like British horror. There're quite a few grisly moments as well.