With the help and encouragement of Brian S I have been looking around The National Archives (Kew) website, trying to get a handle on the filing systems etc. I found this page invaluable: Detecting your browser settings
I have also just ordered this book as I've been told it's a very handy guide: Amazon.co.uk: Air Force Records: A Guide for Family Historians: William Spencer: Books There is another for the Army: Amazon.co.uk: Army Records: A Guide for Family Historians: William Spencer: Books