Now being shown everyday on Yesterday.
Setting three series in a claustrophobic prison planning escapes doesn't sound very promising and more suited for 90 minute film but this series so far has been totally gripping.
The writing is top drawer especially John Brason who went on to the similarly classic Secret Army. The cast are terrific as well (including David McCallum who I've never been struck on before).
Even the German characters are largely free of heel clicking Herr Flick clichés and their political in-fighting to keep the Gestapo at bay is just as engrossing as the allied POW storylines.
Little British made TV seems to match this high standard of writing at the moment and have watch Mad Men for something comparable.