Glenda Jackson tackles dementia as she returns to TV screens for first time in 25 years

And the 83-year-old vividly remembers witnessing how these "terrible illnesses" wreak havoc upon people and their family and friends.

She once met a man and his daughter, "and he had been abusive and shouting", recalls the formidable octogenarian, whose most famous roles include Seventies films Women In Love and A Touch Of Class.

"She said to me, 'Never in my life did he raise his voice to me; never once did I see him like this'. And it is that that struck me most markedly, because we all have those people inside us."

This month sees Jackson play Maud, a woman with dementia, for moving new BBC drama Elizabeth Is Missing.

The feature-length adaptation follows the determined grandmother's mission to discover what's happened to her friend, Elizabeth, who has suddenly disappeared.

Flashbacks are cleverly woven into the plot, detailing another unsolved mystery - what happened to Maud's sister, who went missing when she was a teenager?

It's heartbreaking, but also poignantly humorous, as Maud tries to discover the fate of both women before dementia results in the clues being lost to her forever.

We also see the impact the disease has on her family, particularly daughter Helen (Helen Behan) and granddaughter Katy (Nell Williams).

One piece of research the two-time Academy Award winner found particularly helpful before taking on the role was speaking to a doctor who works with the Alzheimer's Society.

"I said to her, 'What is it that causes this woman who, in her pre-illness life, is living seemingly quite roundly, and suddenly there are these excessive outbursts of rage?' The doctor said, 'It's frustration.'"

How challenging was Elizabeth Is Missing compared to previous projects?

"I'm very subjective when I see a film for the first time. I tend to sit there and castigate myself and say, 'Why in the name of all that's holy did you choose to do that?' I don't mean the subject, I mean my choices in acting.

"But I must say it is a privilege to be offered work, because to be dealing with something that's so fascinating and important as this, and to work with these kinds of people. Talk about cherry on the top of the cake."

Not only did Jackson think the script was "remarkable", the role appealed because it explores an issue she has been "banging on about for at least a decade".

"We are living longer and this is a deep, black hole, which none of the political parties has really taken on board. Services are not being properly financed; how one deals with this illness, as a society, is still fairly down the political ladder."

It was in 1992 that Jackson became Labour MP for Hampstead and Highgate. The seat later became Hampstead and Kilburn, which she won at the 2010 General Election with a majority of just 42.

She stood down from parliament in 2015, after deciding not to put herself forward for re-election.

But the opinionated star is more than happy to discuss her political thoughts ahead of the General Election.

Brexit is the only thing the parties themselves seem to be concentrating on, she notes - even if the people of this country want to discuss issues which are not the UK leaving the EU.

"I have a sneaking feeling that, once December 13 arrives, we'll be in exactly the same position as we're in now. But it's up to us really, as a society, to keep pushing for more reality."

As Jackson points out, the problem is that looking after people with health issues such as dementia requires "training people who know what they're talking about and funding their training and actually providing the care for the people who, at the moment, find it impossible to fund it for themselves."

"We all know the stories about families who are having to sell the family home to be able to pay for someone to be cared for, and this, in itself, creates terrible dramas in families."

She adds: "So, it's a big - as I've said more than once and will continue to say - black hole for all of us and we just have to be brave and find a candle and start working our way through it."

Elizabeth Is Missing is on BBC One, Sunday, 9pm