1970s horror (maybe late 60s) set in a tower
 

As a kid i remember seeing this very disturbing British horror flick, which looked like it could have been from the Hammer or Amicus production teams. It featured a bunch of teenagers in an old lighthouse or tower. I also remember a rather graphic sex scene.
Can anyone shed some light on this? I'd love to know.

with the incredible Robin Askwith...

http://moviesondemand.virginmedia.co...ow_cleaner.png