I have heard/read this version, I think from Tom Bower. Happy to be corrected if pps know more/different:
Prince Charles, Camilla, HAM and possibly some junior royals were at the GP, where the whole point is for them to wander through the crowd and speak to the guests. A special highlight, which the guests were excited about, was the chance to meet newly weds HAM. They arrived late…I think and then M said something like “We’ve shown our faces - come on, let’s go - this is boring “. They then approached Prince C and Camilla who appeared to try to dissuade them and urge them to stay. This was on film, btw, though I can’t vouch for their exact words.
in an odd exchange, M ends up jokily sticking out her tongue at Camilla - all in full public view - at which point Charles approaches them and says something, possibly telling them to go.
it used to be all over YouTube, but I’m not sure it will still be there.
What would QE2 have thought about a royal sticking a tongue out at another in front of the guests? Wow!