Hello
Great success and disappointment best sums up the place. You find a brief reference to your war incident of interest in a file with an original file reference. However, the original file is simply not in the paper indexes or on Discovery, or the file is held there, but the Report or part you want has been removed, per Minute Sheet.
Make sure you have all the correct documentation with you, to prove who you are and where you live, to get a Readers Ticket, to see original documents.
http://www.nationalarchives.gov.uk/about/visit-us/http://www.nationalarchives.gov.uk/about/visit-us/researching-here/do-i-need-a-readers-ticket/If you are looking for WW 2 Naval stuff and it does not appear in the catalogue, you have to search in large Admiralty Digests (by code numbered subject headings) first and hopefully get an original file reference or War History Case number, which you have to convert to a current catalogue reference.
Lots of stuff, including individual incidents, are often hidden behind a short file title.
Mark