One has to remember that certain things that happened during WW2 are still covered by the UKOfficial Secrets Act and those most sensitive matters will be under a 100 year classification. In theory there should be some very, very interesting material released into the public domain in 20 to 25 years time. How much material material "mysteriously gets lost" before it is released remains to be seen. I would imagine that not all material currently covered by the Official Secrets Act will be released even after 100 years if it is felt to be too embarrassing or damaging to the UK's national interests.