But this may top them all. But for very different reasons.
It's not from any stage musical she was appearing in on the West End, but rather a London concert that got aired here on PBS in 2002. The show was One Enchanted Evening, a tribute to Rodgers & Hammerstein. I saw this when it was first broadcast, and the show was fine, but it was this number in particular that blew the usually-solemn British audience out of the water, and almost had me on the floor in hysterics.
The performance is a...well, no, I'm not going to say a further word about that, and give anything away. You can enjoy it just like the theater audience did. I'll just say two things -- 1) Listen carefully for a very funny twist on the lyrics at the 1:00 mark. It's very subtle, and the audience misses it, but you can tell by Dame Judi's reaction that the one-word change is there. And 2) it's hard to imagine many highly-respectable actors having even half as much ludicrous fun at their own expense as she is.
The actor opposite her is Brendan O'Hea, who had earlier appeared with her in that aforementioned revival of A Little Night Music as the ever-moody, young Henrik.
And with all that out of the way, here from The Sound of Music is "16 Going on 17." The video quality isn't great. And you won't care.