My agony at the day my daughter, 20, went for a hike on holiday in the South of France - and the terrible twist of fate that meant she never came back

This summer, I should be planning an incredible getaway for my family of five, a precious chance to spend time together amid the general busyness of life. My twins, Clarissa and Ollie would both be 22-year-old university graduates, looking forward to their new lives, and we'd be planning to wave my youngest Hugo, 20, off on sabbatical to America.

But all that changed on May 7 last year, when my beautiful daughter Clarissa died on a mountainside. One moment she was there, happily chatting as she walked up to a summit in the Gorges du Verdon in central Provence, the next she was gone, having fallen back into her friend's arms.

She shouted, 'Oh no!', tried to scrabble to her feet then went out like a light, a cardiac arrest suddenly stopping her heart for ever. Nothing — not CPR performed by an American woman out hiking, or a shock to her heart by the medic who arrived by helicopter — could save her.

Earlier that day Clarissa had stopped to take a swim in a lake. There is a picture of her standing facing the sun, drinking in all the beauty around her. It's impossible to believe looking at that picture that just a short time later, she would be dead.

Clarissa Nicholls died of a sudden cardiac arrest on a mountainside in the South of France

Clarissa's death utterly destroyed us and every day is difficult, writes Hilary Nicholls 

Clarissa was on a weekend break from her internship at a publisher in Paris, where she was living for her year abroad during her languages degree at the University of Cambridge. She'd completed a stint in Bologna, and had settled into life in the French capital well. It's no exaggeration to say hers was an 'Emily in Paris' lifestyle.

Her flat was in a wonderful old building in the 11th arrondissement. There was a lovely bistro across the road, and every day she cycled off to work at the French publisher Flammarion, on a beautiful street with a view of the Eiffel Tower. She and her flatmate spent weekends rummaging through stalls at flea markets, meeting handsome young men for dates in bars and hosting a long list of friends over from England.

The only blot on the landscape was that she had suffered from bloating issues for six months. That was the only health problem she thought she had.

Clarissa was incredibly sporty, had been a keen runner from an early age and played hockey at Cambridge. But in April 2022 she had a fainting fit which lasted ten minutes after playing a game of Spikeball, a team game with four players hitting a ball with their hands into a round net.

Her friends had been sufficiently worried about her to call an ambulance and she was rushed to Addenbrooke's Hospital. But as far as we knew the ECG of her heart was normal and when we followed up with a neurosurgeon they could find nothing wrong with her.

She had her whole life ahead of her, full of possibilities and friends. I'd always described Clarissa as being like a lint roller: she was someone who could gather up people wherever she went. She loved the sun, whipping out a bikini to top up her tan at the slightest hint of heat in the air, and was also the light in many people's lives.

Back home in London we were getting ready to visit her the following weekend for her 21st birthday celebrations. Our last conversation had taken place the weekend before. We used to speak once a week. She was fiercely independent and had been since she was a little girl, but unusually she rung me twice in two days before she left for her trip, asking me what shoes she should wear for the rocky terrain.

I hold onto that now, wondering if she had a sense that something bad about to happen.

But I wasn't worried. When I hadn't heard from her all weekend it was in the back of my mind that she might call soon, but that was it.

So when there was a knock on the door on Sunday evening, I was surprised. When we opened the door to the police, my first thought was not that there was something wrong with one of the children, but that I'd done something wrong. Not paid a parking ticket, or similar.

Clarissa in Paris, where she living for her year abroad during her languages degree at the University of Cambridge

Clarissa (back, second from right) was incredibly sporty. But in April 2022 she had a fainting fit which lasted ten minutes after playing a game of Spikeball, and her friends had been worried 

It's hard to explain the sheer shock and disbelief you feel when a policeman tells you your child has died. The first word that comes to your mind is 'No'. No, this can't have happened to Clarissa, this can't be happening to us. My husband Simon burst into tears, but I froze. Your next thoughts are of your other children: how are we going to tell them their sister has gone? Our life had been ruined in an instant, and we were about to ruin theirs as well.

Mixed in with those emotions was a sense of urgency. We needed to get to Nice as soon as possible because under French law, they seal the casket of someone who has passed away after 48 hours. It was not just a question of breaking the news to the boys but also getting them on a plane. That was more straightforward with Ollie, who was studying in Oxford, about to take his Finals. The police blue-lit Simon up there, and Ollie's reaction to the news was like mine — a struggle to believe what had happened.

But Hugo was in Ireland, reading business and economics at Trinity College, Dublin. Late in the evening, I had to video call him but made sure he was surrounded by friends. He said, 'Oh no!' and fell to the floor.

It's no exaggeration to say that call was the hardest I have ever made. I wanted to be there to hold him, as Simon could be with Ollie. Instead, I had to rely on his friends to do that for me.

It was hard to persuade Hugo to come to France with us, I think because he didn't want to be confronted with the pain. But for me it was important we went as a family. I didn't want anyone to regret not saying goodbye to Clarissa one final time.

The flight to Nice was awful. Then the tears came for me as well. I don't know what the people around us thought, but I will never forget the kindness of one woman who silently handed me her handkerchief, without need for explanation.

At 7.30pm that evening we saw Clarissa, so beautiful lying there at peace without a mark on her. You expect there to be some sign of death, some reason why, but there was none. We hugged each other and cried.

The following weekend, instead of visiting Clarissa to celebrate her coming of age, we went to pack up her belongings. Clarissa had bought a deckchair in a fleamarket so she could sit on her balcony and soak up the sun, it's now at our London home. It's those small things which keep the connection.

We took Ollie with us, who'd decided to delay his Finals. As her twin, it was still his birthday, so it was important to all of us to go out to dinner together even if there were a lot of tears.

Clarissa was flown home the same weekend and we started to organise her sending off, on June 27 at Farm Street Church in Mayfair where Simon and I got married. But as well as making sure we said goodbye and celebrated her life in the way she deserved, we also wanted answers.

First, we had her heart checked by a pathologist who confirmed she had an undiagnosed heart condition called arrhythmogenic cardiomyopathy. A normal athlete will strengthen their heart but in someone with this condition, exercise will make it worse, like straining a pulled muscle.

We've discovered since then that more than 600 young people die from a sudden cardiac arrest each year (that's at least 12 a week) and 80 per cent of young people have no symptoms at all. Many are found dead in their beds by their parents.

Clarissa had been a keen runner from an early age and played hockey at Cambridge

The family found out later that Clarissa had undiagnosed heart condition called arrhythmogenic cardiomyopathy

We have to bear the fact Clarissa did have a warning sign: a cardiologist confirmed her Addenbrooke's ECG did show signs of the condition. We don't blame the hospital — it would have taken a trained cardiologist to spot the signs. However, Addenbrooke's has now changed its processes to make sure someone presenting with the same symptoms as Clarissa will have their ECG read by a cardiologist.

Had we discovered it in time, Clarissa would have probably had a defibrillator fitted and not been able to do more than essential exercise.

Losing a child can put a marriage under strain. I've experienced moments with Simon when he's been grieving and I've been getting on with things and then the reverse happens. But I'd say our marriage is the strongest it's been. He's never wavered from being by my side and I see the goodness in him like never before.

Together, we want to create a legacy for Clarissa. We are mindful of the fact the country is losing a secondary school's worth of wonderful young people to sudden cardiac arrest every year. That affects not only their families but all their friends.

I've thrown myself into campaigning for the charity Cardiac Risk in the Young, lobbying government to extend screening to 14-35 year olds who participate in sport, rather than just those with a family risk. Her friends fundraise for Clarissa's Campaign for Cambridge Hearts, getting sponsorship from local businesses so we can screen young people in the city.

Clarissa's death utterly destroyed us and every day is difficult. But we find a way to put one foot forward in front of the other for Ollie and Hugo, who have their lives ahead of them.

We've tried to gather up Clarissa's friends around us. On the anniversary of her death, we did a tour of the trees we planted in her memory — an oak tree at her school St Mary's Ascot, and a magnolia at Trinity Hall, Cambridge — and went out with her friends. For the anniversary of her funeral, we'll have an annual celebration with a theme chosen by a different cohort of her friends. This year it's 70s groove.

That connection is all important for us and the boys. I'm not sure if it is a good or a bad thing but I've left no stone unturned in my bid to understand her life and as a result I've met dozens of people who were close to her. It keeps Clarissa's memory alive for us. I try my hardest not to fear for the future, for Hugo going to California or for Ollie as he steps out into the world. It's now just the four of us, but Clarissa will always be part of our lives.

For more information about screening see testmyheart.org.uk and Clarissa's memorial page www.c-r-y.org.uk/

As told to Lebby Eyres