The Luftwaffe traffic being deciphered by the British in 1940 was growing in volume but patchy nevertheless. Large numbers of messages would be picked up from some units and HQs, little or nothing from others. I’ve only recently started looking at the relevant files, prompted partly by my annoyance at some of the TV programmes made for the 70th Anniversary of the Battle of Britain.

There is an introduction here to the form and content of the material on file at the National Archives, or you can go straight these articles I've based on it: