As I know that WR knows, there is a big problem for researchers into the more sensitive issues re. WW2 history (and other aspects of British history.
1: A lot of sensitive stuff was deliberately not kept.
2: Some files which were kept have had "interesting" histories, some being "contaminated by asbestos" or "damaged by water" and hence destroyed, despite being slated for eventual release. Some doubt over the veracity of these claims have been cast (i.e. open ridicule). The prime example being the majority of the archived SOE files. These were originally declared lost (I think in a fire in the old building ???) but all of a sudden they turned up a few years ago. They are not fully open, but SUPPOSEDLY even the closed ones are mainly available to accredited researchers.
3: Dates for release. Originally most files were on 30 year release dates (which explains the big releases of WW2 info in the mid-70's. This is now meant to be being reduced to 20 - BUT at the same time the areas of "no-go" info (e.g. NBC) is enlarging leading to big gaps. In addition to the 20/30 year restriction there are other ones 50, 70, 100 years. These are partly due to genuine secrecy, partly sensitivity (operational cock-ups, etc...) and what is sometime seen as a particular British sensitivity particular names. ("The man who never was" release of the name of the corpse was a VERY rare exception.)
AND TWO MORE "PROBLEMS" ;
4: Some archives are underfunded and hence awkward, if not actually difficult, to access. The prime example of this is the Royal Navy Historical Branch which was reasonably helpful when in Whitehall, but since it has gone down to Portsmouth they are near enough impossible.
5: AND THE BIG ONE - The Foreign Office Archive. This was a secret until about 4 or 5 years ago. It supposedly has miles of files from all ages and can be expected to contain a lot of fascinating info. The FO originally said that they would open as much as possible of the archive as quickly as possible, but that pledge SEEMS to have evaporated. It is possible to get in to research, BUT you need to be a known researcher and be found acceptable. I have no idea of the qualifications to get in, but it seems to be quite difficult.
Good luck to ALL researchers.