The nerves are partly to do with the fact that he finds it nearly impossible these days to remember his lines. He has touched on this briefly in the past, but it is clearly a source of real anguish and his eyes fill with tears as he talks about it.

The stage, he says, is his true passion and yet: “Now I can’t really do theatre. It breaks my heart, but I can’t really remember lines. That was my whole life. I did more than 20 West End plays...”

Television is easier “because I have an earpiece. You can’t do that on stage [and] I find I’m trapped without it.” The Casual Vacancy was filmed with someone reading Gambon his lines through his earpiece – a practice common among American actors, but less so here.

“It’s a funny game,” he concedes. “I don’t know how to cope with it.” Are there medical reasons for his memory loss? “I think it’s just age. Your brain doesn’t work as well as it used to. It affects memory and that’s obviously connected to speech. I suppose that’s my problem.”

He looks bereft.  “My emotions are coming out all the time,” he says. “I’m always on the edge. I wasn’t a bad actor, because I’m quite an emotional person... My friend died a few months ago. They had a funeral and I was meant to speak and I couldn’t.” He motions toward his eyes, which are still glistening. “It’s weird, human nature.”

Acting is his great love. He refuses to talk about his private life, even to confirm whether or not he is married (the cuttings suggest he has been – possibly twice – and has three children), but he is unstoppably loquacious when it comes to talking about his profession.

He says he always liked the idea of acting. He was born in Dublin, to Irish parents who moved to Camden, north London, when he was five. One of Gambon’s earliest memories is of standing by the platform at Euston train station with his best friend, Kevin, in the hope that a driver would lift them up into the truck.

“The heat, the noise, the smell was overwhelming,” he recalls. It helped “if you made yourself look really pathetic”. This, then, was his first experience of acting a part and seeing instant, satisfying results. 

Although he later trained as a toolmaker, Gambon knew he wanted to act, and wrote a lengthy letter detailing an imaginary theatrical career to the head of the Gate Theatre in Dublin. He was taken on, after which there was no look- ing back. In his early 20s, he returned to London for a stint at the National, then moved to the Birmingham Repertory.

His parents, Edward and Mary, came to see him there once: “They stood at the stage door at the end and they could barely speak. They were just staring at me because it was something to them that was completely another world...  “I’d left home with a strong Dublin accent and then they saw me on stage and my voice had changed, my attitude to life had changed and my outlook had changed. I was quite upright and quite posh and it overwhelmed them.

It is probably a mark of his innate natural talent that his Wikipedia entry erroneously states that he went to Rada: “If you could point out I didn’t, that would be useful, thank you.” He was knighted in 1998, a fact Gambon remains unfashionably delighted about. “A well-known actor, a friend of mine, rang me up and said, ‘Say no. Tell them to f *** off!’ ”He laughs. “And I was feeling quite weepy about it.”

He “never uses” the title, “because I’d feel like an idiot, but it’s a nice thing to be given”. These days, Gambon is worried that there are “many more actors now than ever before”. Someone told him recently there was a drama school in every British town. “And I thought, ‘God almighty!’ It can’t be really right, can it? Because you’re leading people up the wrong path, aren’t you? I worry about them, because it must be miserable for them. There must be so many actors who go to drama school and never get a job.” 

What does he make of the current influx of actors who went to public school, including Benedict Cumberbatch (Harrow) and Eddie Redmayne (Eton)? “The more Old Etonians the better, I think!” he says cheerily. “The two or three who are playing at the moment are geniuses, aren’t they? The more geniuses you get, the better. It’s to do with being actors and wanting to do it; it’s nothing to do with where they come from.”

He insists that class isn’t an issue any more, so much as money and the unequal distribution of it. Still, there is one area in which he openly defines himself as “a snob” and that is reality television. I’d read that Gambon had an interest in ballet and, indeed, there is an unexpected grace to his movements – when he flicks his hand to illustrate a point, he resembles nothing quite so much as an elegant maiden aunt summoning a butler in an Oscar Wilde play.

So would he ever consider appearing in Strictly Come Dancing? “No!” he says, almost shrieking. “I’m a terrible snob about it. I think, ‘Oh, that’s a bit common.” He giggles. It is a line delivered with all the magnificent panache of Lady Bracknell herself.