Rapaire,

"...In 1963 I saw a British film based on the the Great Escape premise that totally flattened "The Great Escape" (which I saw later). Can't remember what it was, tho.... "

Could the film have been "The password is Courage"?, Dirk Bogard as one CSM Coward, leading a mass breakout from a POW camp? (Same tunnel-stops-short-of-the-tree-line plot twist).
Do you remember a bit where a saw mill burns down or a train is derailed at some point?

Walrus