Yes there's a lot of looseness. Considering the history of the D-notice in wartime (1914-18 and 1938-45), when the stakes were high, it's a typical British implementation of "strong flexibility". The problem may actually be over-compliance. The days of Woodward and Bernstein have given way to journalists whose idea of ambitiously advancing their careers involves not offending anyone and running to mummy to ask permission at every juncture - not of government but their corporate/advertising lords.