Sue Bourne and her 83-year-old mother Ethel are sitting in Ethel’s bedroom in a care home in Ayr, discussing how she spends her time. The two women share the same gentle Scottish accent and crisp, precise diction. Sue lives in West London, and it has been a source of some guilt that she manages the 800-mile round-trip only about once a month.

“I talk to my dad a lot,” Ethel says wistfully, nodding towards a framed black-and-white portrait of a middle-aged man.

“He’s not your dad!” Sue exclaims incredulously. “That’s my dad. Your husband! Who you were married to for 50-odd years!”

“Oh, that’s why he’s so familiar!” says Ethel, and bursts out laughing.

There are many moments like this in Mum and Me, Bourne’s video diary of three years visiting her mother, who was found to have Alzheimer’s six years ago. It is certainly one of the most affecting documentaries I have seen for some time, often poignant, occasionally heart-wrenching, but mostly just sweet and very funny.

“Alzheimer’s does seem to be on the increase,” says Bourne. “I suppose it’s because people are living longer. I knew the film would strike a chord with people because everybody knows somebody. We’re all affected by it now.”

The disease has certainly been eating steadily into media schedules. Iris Murdoch’s illness was portrayed in Iris in 2001. Three years ago Richard Briers played a pensioner caring for a wife afflicted with Alzheimer’s in Dad. Frank Finlay had it in Life Begins.

Last year the furore over the supposed death scene in Paul Watson’s second documentary about Malcolm and Barbara Pointon, Love’s Farewell, drew distracting negative attention to a stunningly powerful film. This year the author Terry Pratchett has “come out” as a sufferer.

Alzheimer’s, or senile dementia, even features in another documentary this week. In Marilyn Gaunt’s Class of ‘62 (Mon, BBC Two, 9pm), which updates the lives of the same group of friends she has been filming at intervals since they all left school 46 years ago, several of the women’s lives have been severely affected because they have had to care for suffering mothers.

“It’s usually the women,” Bourne reflects. “The men tend to die first.” She originally filmed her visits to her mother as a record for herself and her teenage daughter, Holly, but the very bleakness of most of this other material spurred her to go public.

“Every documentary I’d seen about Alzheimer’s was just grim,” she tells me. “Paul Watson’s film was wonderful, but, by God, it just put the fear of God into you. I hadn’t seen a film that was hopeful. Me and my mum can still have a laugh together and have a relationship. I think she’s exceptional – we have fun!”

Even the saddest moments in Mum and Me are relieved by Ethel’s apparently inextinguishable sense of humour. On a rare occasion when Sue and Holly arrive at the home to find her dejected, she tells them tearfully that “I just realised that my daddy’s dead!” “He died 50 years ago!” says Sue, and, yet again, they both collapse in fits of laughter.

“Initially I thought the film would become the journey of how I fell in love with my mum,” says Bourne. “We had a difficult relationship. I got on with my dad, but I didn’t get on with my mum, but she’s become so sweet in her dotage! I have sort of fallen in love with her. She’s taught me and Holly that whatever happens you make the best of it.”

Now a student at Glasgow University, Holly is central to the film, both for her special bond with her grandmother (“I love you granny, but you’re nuts!”) and as the camera operator.

Despite an impressive portfolio as a producer and director, Bourne had never used a camcorder and some of the earlier footage is decidedly shaky as the two women learn a new craft.

“Mum has a very healthy appetite,” says Bourne. “And we always take her out to cafés and restaurants, but it can be tricky setting up the camera in the car. Sometimes we had to prop it up on a pile of incontinence pads.”

The film certainly goes further than most in showing the difficulties of looking after an incontinent. Bourne is also disarmingly honest about the strains of the situation for all concerned. Carers everywhere will recognise the guilt, exasperation and sheer weariness that occasionally afflict her.

She knows her mother will not be like this for ever and dreads the day she will have to make that long journey “to visit a vegetable. That sounds terribly unPC, doesn’t it?” Well, no actually. It does sound completely honest, though – like this humane and uplifting film.

Mum and Me, Tues, BBC One, 10.35pm (N. Ireland: Wed, 11.55pm)

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