So are you saying that if you serve a member of the Royal Family a drink, you might well forget that fact 10 years later? Course you'd remember, it's not like the general public bumps into members of the RF as a matter of course!
I've seen two members of the RF (didn't go looking for them, they were both just there !) but I remember it distinctly. One was PAnne and one year (and one year only) I went to a horsey thing and she was there. It is really easy for me to find out when that was, I keep diaries!
Of course, working in Pizza Express might be different, but it's hardly like he goes there a lot! So if the waitress saw him there, it would obviously be that date.