Penny wrote of Julia Cleverdon, who worked for Charles for more than 30 years, and who described what life was like for Charles and Camilla in the aftermath of Diana’s death.
She said: “That must have been an appalling period for her, and I thought she was courageous and humble and decent.
“He was in agony about it – he really, really was.
“And the fact that she was prepared to go through all this for him, I think, illustrates what an incredibly strong relationship and partnership they’ve had.
“Knowing a bit about it at that time, when we ever talked about it, he was in genuine agony about what she was putting up with.”
The Duke and Duchess of Cornwall may well be one of the royal family's most enduring love stories.
After meeting in the 1970s and marrying other people, the couple finally tied the knot in April 2005 - despite divorcing their previous partner years earlier.
The Queen gave her official permission for their marriage in February of that same year.
In a short statement, she said: "The Duke of Edinburgh and I are very happy that the Prince of Wales and Mrs Parker Bowles are to marry. We have given them our warmest good wishes for their future together."