Again, Alastair Sim manages to impose himself on the cast of this superior British espionage yarn and turn it into something surprisingly enjoyable. It's a tale set in a tiny wartime Scottish hospital were a Nazi spy ring are trying to get the secrets of local scientist Leslie Banks' innovative new bomb sight. Soon, though, we discover that the authorities are aware of the goings-on and have planted a counter-agent - we just don't now who is who! There are plenty of decent performances from Sim; John Mills as recuperating RAF officer "Perry"; Michael Wilding; an on-form Catherine Lacey as…