Help finding similar movie

I am trying to find a somewhat old, b&w, WWII POW movie that may be about Colditz - or another camp perhaps.

I have watched the John Mills film recently but it doesn't seem to be the movie I remember.

What I do remember:
- there are secret maps rolled up and hidden inside pencils
- there are secret messages under the labels of their phonograph/78 records
- they secretly build a kite and fly out of the attic of the building

Any help appreciated.