Dame Maggie Smith flops her head down onto the desk with a dramatic flourish, writes Keeley Bolger.

Neatly turned out in a pale green jumper and leopard skin print scarf, the theatrical gesture is the 80-year-old star of the stage and screen's initial deadpan response to how she remains energetic during busy bouts of work.

"The energy comes from the people around you and one's director," she says in her trademark crisp tones.

"But it is tough and I can't say it's easy at the age I am."

Born in Ilford, Smith moved to Oxford when she was four, joined the Oxford Playhouse in her teens and has been steadily working since.

She won her first Oscar in 1969 for her leading role as spirited teacher Jean in The Prime Of Miss Jean Brodie, and her second for her supporting role in 1978 comedy, California Suite.

Then of course, there's her role as the firm but fair Professor McGonagall in the Harry Potter series and lately as Downton Abbey's scene-stealing snob, the Dowager Countess Violet Crawley.

While there is much debate about the lack of roles for older women, Dame Maggie concedes that she is "lucky" with the work she is known for.

"There's always this endless thing about how there aren't any parts for women over a certain age," she explains.

"Well, I've kind of reached the limit now, because I'm sure there aren't many [parts] after the age I am, but I think they're talking about middle age."

Although she's keen to keep on working, some of the physical aspects of the job are harder than they used to be.

A recent example of this is her role as the eponymous character in The Lady In The Van, which is based on the real-life story of a transient elderly woman, who parked her car in playwright Alan Bennett's driveway and stayed there for 15 years.

When Bennett first moved into Camden in North London in the late Sixties, the woman, who he came to know as Miss Shepherd, was already living in the van, although further up the street.

He gradually became aware of her, as she and her van, which was dirty and filled with shopping bags, old bits of carpet and rubbish, drifted down his street as she systematically outstayed her welcome outside every other house.

Although not friends in the strictest sense, the two formed a bond and their relationship was played out in Bennett's 1999 stage show, based on his memoir, in which Smith starred as Shepherd.

Clearly, the actress, who lives in Chelsea, has empathy with Miss Shepherd's plight.

"Loud and ghastly though she was, there's a terrible tragedy there," says Smith.

"She isn't subtle, she just isn't. She drove them mad, up and down the street, although they were much too nice to say. And they wanted to keep up with Alan I suspect, to make themselves look as tolerant as he was."

While the play was "demanding", she could "handle" the intense workload at the time.

The film, however, was "very, very different".

"Miss Shepherd was much broader and the film was much more concentrated - it was a whole different thing... the stage was more demanding and it was a long time ago," she says.

And she found herself out of her comfort zone the second time around.

"It wasn't easy being constricted by the van," she laughs. "When it was dry, it wasn't so bad, but it was wet most of the time because of the rain.

"It was where I spent most of the time, I must admit but they [the rest of the cast and crew] were all in comfort in the house. They were in luxury. Figures, doesn't it?"

The filthy van, which according to the director Nicholas Hytner, was broken into one weekend and used by a couple, "having a good time with each other using whatever substances," was a sharp contrast to Smith's TV home in the grand grounds of Downton Abbey.

Along with the rest of the cast, the star completed her last scenes in the landmark ITV series earlier this year, but she wouldn't rule out a role in the film version if it does happen after the Christmas special.

"It would be fun," she smiles. "I'm not sure if the wig will be around, but hopefully I will be. I think the wig is slightly more tired than I am.''

Although she's not "very close" to either the Dowager or Miss Shepherd in personality, it was a nice change to play a less buttoned-up character.

"I can only talk as an actress, but it was much easier to be Miss Shepherd, because she didn't mind how she looked," says the mother-of-two.

"It was such a relief, because Lady Violet was forever in those corsets that Miss Shepherd would never have dreamed of going near. So for comfort alone, it was nicer to be Miss Shepherd."

Although Smith jokes about the van, on a serious note, she notes how fortunate she is to have a roof over her head.

"I'm so glad I've got a home," she says.

"Having had a practice of not having one, it means an awful lot. There is no way you could live in a van. I know some people do, some people live in cars, but surely not for that length of time.

"I've lived in my home for a very long time and I dread to think what it would be like without it. It means everything."

The Lady In The Van is in cinemas now