Visibly shocked and surprised to be handed the award, it was proceeded by a short film about her career featuring contributions from her many co-stars and fans including Jennifer Saunders, Coronation Street's Bill Roache, prime minister David Cameron and Wolf of Wall Street director Martin Scorsese.
Speaking of her secret award, she told journalists backstage: "I've got to say it's the most unexpected and sort of earth-shattering thing that's happened to me.
"To see those kinds of people talking so generously about me - excluding Ms Saunders! - and to see my life coming back again, and to see me at 18 diving into a river in a Nimble advert right up to Patrick Macnee to the prime minister, who's taken time out of his busy life.
"Martin Scorsese, who I've only just worked with talking properly about shows that I was in, Ruby Wax, who's one of my great icons, Bill Roache - I mean, I nearly married him! I'd still be in Corrie now if he'd let me marry him.
"The whole thing was like a trance. I just sat there with my son beside me and I'm afraid to say tears were coursing down both our cheeks. It was unbelievably touching. And they showed the northern lights as well, I just [choking up] damn, I'm sorry, I just love you and I'm so touched."
When asked what makes an awards ceremony such as the NTAs special, she said: "I've always had a great passionate and undying and in fact growing love for people. I just love people and what we do in the acting world is read people written by people acted by people for people.
"It's a strange and dull way of putting it but everything we do is about connecting with people. A lot of acting is artifice and a lot of it isn't, and I think it's just very thrilling to be at something as important as this where what we call members of the public - when actually all members of the public are actors as much as we are - recognise the work that other people do to entertain them.
"So I just love humankind and I love us and I love tonight more than I can tell you - thank you."
On whether she still ever gets nervous about her work, she said: "No, not about my work, because I prepare properly and I love it. I love doing it and once you've done something, all of you know this, when you've done something many times - when you take your first picture, you're anxious it's going to be in focus, you're not going to be able to find it properly.
[Joanna Lumley in The Making of a Lady]
"When you've taken 140,000 you know pretty much that you're on the money - not that you're going to be successful, but that you're not going to let people down. So as an actor you just prepare the best you can and do it."
She added: "What touched me tonight so much was [choked up again] seeing a picture of my daddy. He's dead now and was the inspiration behind me supporting the Gurkhas as much as I do. My parents have gone but I've got them here. I don't always cry like this - I never cry like this actually."
Photo gallery - National Television Awards winners 2013: