Up until this time the word grass (def. to inform upon, to denounce to the authorities, to sneak, to snitch) was always used pejoratively. In post-war popular literature informers were those who, at school, "told" on their fellows; or who in wartime would betray friends and neighbours to the occupying authorities (see Myths of the Maquis 1975, Hoorah for the Dorm 1962).
By contrast the verb to whistle-blow (to make public, to bring to light), generally denoted an admirable act. It would be seen as the democratic duty of a public-spirited citizen, safeguarding the commonwealth from the closeted mismanagement of the powerful.
This changed when Stephen Dorrell (see Fifty years in Parliament - the life and times of Mr Dorrell 2036), then Health Secretary, introduced plans to identify incompetent doctors. These included the requirement that practitioners and surgeons inform management when their colleagues were under-performing (see also They Took Off The Wrong Leg, 1997, Hilarious Medical Mistakes (video) 1999). The practice of grassing had thus become an act of professionalism. Contemporaneously, the act of informing the public about mismanagement (whistle-blowing) was made contractually illegal by much of the health industry (then known as the NHS). Such action was deemed unprofessional, counter-productive and uncommercial.
The second event was the crackdown on benefit and other fraud by the Labour-run council in Reading, Berkshire. Where previously left-of-centre politicians (sometimes known as "socialists") had regarded such fraud as an entrepreneurial form of redistribution in the tradition of Robin Hood, Reading's councillors demanded that tenants and others inform on such activities. This was to be seen as a public duty. A year later the reformed administration in Walsall, West Midlands, recommended the demolition of council homes adjoining a fraudster's domicile, to encourage grassing.
Finally, in the last years of the century, Michael Howard, leader of the Conservative Party, advocated a "good grasser's charter", detailing the 20 un-British activities that neighbours should look out for. These included suspiciously opulent motor cars, unexpected children and duskiness.