I have always gotten the impression from biographies, that Charles was somewhat smitten with Camilla because she was not impressed with his title. Camilla may have seen Charles solely as a friend and some one to make Andrew Parker-Bowles jealous. She was very focused on getting Parker-Bowles down the aisle. Also as the years went by, Charles came to see Camilla as "the one who got away" - but at the time, he was being pressured to consider other women as possible bride and to sow his wild oats. For Camilla, I think she finally realized that Andrew was a serial cheater. As some point they got together, but the extent of their relationship prior to Charles marriage will never be completely known. (As his diaries will not be published in our lifetime) And as for when their relationship resumed, we have only Diana's perspective.