The complete issue of this cover sold out in just 3 days. It features the Stefan Knapp Battle of Britain mural with Polish and British stamps being cancelled in each country on the same day. The British cancel is an Enigma machine.
This cover was issued to commemorate the first Polish event at Bletchley Park to thank the Polish nation for their vital contribution in cracking the Enigma code.
If you see this on ebay or elsewhere for anything less then snap it up immediately.