Hi,

Can someone help by recognising this old film for me? I don't know the title and forgot even the names of the actors!

From the 1970's (I think "74"), set in England. I'm not sure if it was Hammer or not, but the title has lost me. It was on TV only a few weeks ago! It's about a small island with a creepy old light house on it. A group of American teenagers (who were British actors) spent a night on the island and three get brutally killed. The remaining survivor is a girl, who runs around naked and screaming her head off throughout the film.. whenever she's in it!....
A couple of men arrive there and one gets stabbed to death by the girl as she screams in panic. Later she's in a mental hospital, where doctors and police are trying to investigate....

They think she killed her friends the doctor is convinced something else happened there. There is a scene of her being hypnotised and screaming.

He goes to the island along with a few people. Almost all the other people get killed there, and you see a scruffy old mad man living and hiding. Later it turns out he wasn't the killer. It has a couple of twists and a sudden ending. I could go on but you might've given up after reading this and not know which film I'm talking about.
Does anyone recognise it? How can I find out?

Thanks I appreciate any help.