S. J. Taylor, Shock! Horror! The Tabloids in Action (London: Corgi Books, 1992), 267-8.
Standard operating practice is to go undercover to produce the scandals that are the bread and butter of the tabloid press. One of the more common scams is to entrap a prostitute by pretending to be a businessman, asking her to your room and then, after she asks for the money, revealing you are a reporter.
A standard Fleet Street story on hotel hookers goes like this. A reporter is on an out-of-town assignment with a photographer and is looking for a quickie story. They come up with the idea of ringing an escort service, asking for an escort for the evening. The plan is for the photographer to get into the wardrobe, and when he hears the reporter say a code phrase, like 'Darling, you've got lovely knickers,' he jumps out and snaps the girl. Ideally, this takes place at the very moment the reporter is saying, 'I'm a reporter from the Sun.'
So the agency sends a girl to the hotel and the desk clerk calls them and they say, 'Yes, send her up to Room 204.' The photographer jumps into the wardrobe. The girl comes in, the reporter offers her money for sex, the girl says 'yes' and begins taking her clothes off. At which point, the reporter says in a loud voice, 'Darling, you've got lovely knickers.' And nothing happens. So the reporter repeats the code phrase, 'Darling, you've got lovely knickers.' Again, nothing happens. One last time, the reporter shouts out, 'Darling, you've got lovely knickers!' The girl throws on her clothes and flees, certain that the reporter is a sexual pervert. The reporter opens the wardrobe door to find the photographer fast asleep.
At this juncture, the author of the tale takes a swig from his pint, leans forward, and asserts emphatically one of the catchphrases of Fleet Street: 'True story.'