The Press obviously knew about Charles and Camilla from the Tampongate tapes,which were recorded in 1989 and provided to one of the papers shortly afterwards. Clearly a story that juicy was never gonna to stay a secret in Fleet Street,and so it got around l wildfire.
But you know, the fact that the recording was an absolutely appalling breach of privacy meant the papers rightly didn’t publish anything about it. Until “Diana: her true story” was published in 1992. That majored on Charles and Camilla’s affair, which made it known to the general public. Diana was then unmasked as a collaborator of the book, and in December 1993 John Major announced that the couple were to separate. Tampongate was finally published in January 1993, the papers by now having convinced themselves it was in the public interest to publish it.
So yes, the papers knew. But manipulated the public? I would have thought we had a different view of phone hacking and tabloid excesses these days?