What's that b/w WWII movie where a German and some Brits are stuck on a remote island with a lighthouse?

I watched this on TV years ago and can't remember the name. There's a 'baddie' German (possibly waiting for a U-Boat to rescue him or escaped from a sinking one) and a Brit or two. It's lit in a 20s/30s German style (I'm guessing it's a late 30s film), very dark and moody and the upper-hand keeps switching between either side. Sorry, this is very vague.