"A few weeks before he died, I gave Vic a kiss, and he went, 'Oh, that's lovely'. It was a lightbulb moment - like he knew who I was again," says June Mellors, 79. "Those moments break your heart, but make it worthwhile."

Immaculate in a grey tweed trouser suit, June shakes her elegant grey bob in disbelief that her husband, who suffered with Alzheimer's disease for nearly a decade, is no longer with her, after 63 years by her side.

For Sheila Winterton, 75, a similar "lightbulb moment" came on a recent visit to her 79-year-old husband, Derek, with whom she shared 60 years, and two children, before his dementia saw him moved to an Eastbourne care home where he has lived since September. "The other day he put his head on my shoulder," she says, tears filling her brown eyes, "and he said: 'My She.' He's not done that for a long time."

These precious moments - "they're in a fog, but then something gets through to the brain", says Sheila - are cherished all the more for their rarity. They are intermittent beacons that illuminate a shared history and deepen an enduring love.

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For June Aiken, 69, they pop up on trips to the opera and musical theatre with George, her 76-year-old husband, a retired engineer who was diagnosed with dementia in 2009. The couple were stalwarts of the Darlington Operatic society during their 45-year marriage, and she says: "I know we feel the same joy as the uplifting music floods through us both."

Jenny Eldridge, 67, finds such lightbulb moments harder to identify; life with her husband, Mike, 75, her partner in business and - more pertinently - in love for 30 years, is very tough at the moment. But then she recalls: "When he gets dropped home after spending time at the day care centre, he says: 'I missed you, I missed you."' She wells up at the thought.

Sitting in the Langham Hotel, Eastbourne, with Sheila, Jenny and the two Junes, it's impossible not to be moved, not just by their pain, but by their stoicism. The four women have met me here to talk about how it feels to lose your still-living husband to dementia - of the inevitable lows, but also the surprising highs that come with honouring the promise to love someone in sickness as in health.

The four, who will appear in Dementiaville, a three-part documentary on UK's Channel 4, met through a remarkable dementia day care centre called Ivy House in Eastbourne, founded by a psychologist, Jane Lowe - who, the four women tell me, cares as much for the carers as the patients. "I couldn't have done this without her," says June Mellors, admitting she has called Jane at all hours, desperate for advice.

But I believe she could. Because June - with her friends - has been driven to care by the type of love that only seems to deepen through adversity, as we all hope great love should.

We should all heed their stories. There are 850,000 people with dementia in the UK, with numbers set to rise to more than one million by 2025, says the Alzheimer's Society. Will all be as lucky - to be loved and mourned, in life and death - as the husbands of the Eastbourne quartet?

Together, this band of sisters has formed a tight friendship, taking turns to give each other the emotional support needed to survive the daily challenges of dementia's long goodbye and drawing strength from those moments when they glimpse once again - however briefly - the men they fell in love with.

For June, right up to the end, it only took a look. "Vic still had the most gorgeous eyes and smile," she says. "When he recognised me, it would take me straight back to the day we met in 1953, in an ice-cream bar on the Edgware Road, when he mended my bike."

Sheila agrees: "Little things come back and you cling on to those. I helped Derek with his dinner the other day, and afterwards he said, 'Shall we walk down the village?', which is what we always used to do, before he became ill."

Sometimes the magic has to be conjured: June would encourage Vic to dance to his favourite Barry White - "even though he didn't know what he was doing with his feet". Sheila takes one of the couple's four grandchildren to visit, knowing he can connect with them in a way he can't always do with her.

Jenny can still raise a smile from Mike, sometimes - but however many lightbulb moments there are, life while caring for someone with dementia inevitably gets tougher as they slip further away.

"I say give me a cuddle, but he doesn't cuddle back," she says. "But I'm not ready to lose him. I'm desperate for those glimpses of the man I love."

Mike, she explains, used to be smart and dapper, "but now he is unkempt, he often won't shave or shower". Worse, she has to handle his creeping aggression. "He doesn't understand a lot of what is said, and he flares up, threatens me with a fist. Sometimes, I think he is not my husband, but a person I have been forced to care for."

June Aiken admits the roles within her marriage have changed, too: "George needs me to feel secure, whereas it used to be the other way round," she says. "At times, he leans on me like a petulant child, because he is tired, especially at night and I just hold him."

Although the women all admit to feelings of searing isolation and loneliness, especially at night - "I cry all the time," says Jenny - June Mellors is dry-eyed, despite losing Vic to his dementia four weeks ago. "I don't know why I can't cry," she says, perplexed. "I used to cry after visiting him, cry on the way there; if you got a smile out of him, you'd cry, but when he slipped away from us... it's unreal," she concludes. "I feel unreal."

But while dementia may have robbed these four of their husbands, transforming them from wives to carers, the love they shared still, palpably, remains.

"You never stop loving them regardless of what you're going through, and the love gets you through," says June Mellors.

"It's role reversal. Vic looked after me, and then I was the protector of him and his dignity. I couldn't have loved him more. And it gave me a lovely feeling, knowing I was there for him when he needed me."

Sheila says her love for and devotion to her husband, who has been her closest companion since she was 15, has only deepened - "if that's possible'."

"When he was at home, I used to pray to God, 'Take him before he gets too bad'," she says. "But now, I still feel I can do things for him. I shave him and rub cream on to his hands and feet. He worked hard his whole life to care for me, and now it is my chance to do my bit.

"He was my strength, my support, my rock - all our lives," she adds. "Now it's my turn."