Two days into their family holiday in Cyprus, Jed and Julie Donnelly were really starting to relax.  As they lay in the sun, their 17-year-old son Adam was playing volleyball with friends in the shallow end of the hotel pool. 

Within minutes, this idyllic summer scene turned into a nightmare as Adam, an otherwise healthy teenager, suddenly collapsed.  One of his friends ran over to tell the parents their son was in trouble. 

'Apparently he said he didn't feel well, and then just fell forward into the pool,' recalls Julie, 49, a book-keeper for the Roman Catholic Church. 

A paramedic happened to be nearby and she rushed over and tried to resuscitate him.  But it was no good: by the time we reached the pool, it was clear nothing could be done.' 

During the initial confusion, it was thought that Adam had drowned after collapsing, but a post mortem showed he'd died of a cardiac arrest. 

The death of a child is the worst thing that can happen to you,' says Julie.  In fact, the ghastly truth is that the Donnelly's' experience is repeated daily; every week, 12 apparently fit and healthy young people in the UK die from undiagnosed heart conditions. 

These sudden deaths are generally caused by thickening or abnormal anatomy of the heart muscle, or an irregular heart, explains Dr Steven Cox of the charity Cardiac Risk in the Young (CRY). 

'The majority of deaths are due to inherited disorders, ' he says. Often the death is triggered by sporting activities, which put pressure on the heart; athletes are thought to be a particular risk. 

An estimated 200,000 young people (defined as up to the age of 35) have an undiagnosed heart problem that could cause sudden death.  Unfortunately, as Dr Cox points out, many are not given any warning signs. 

Symptoms include exercise-related chest pain, breathlessness, palpitations, dizziness or fainting.  But 80 per cent of the 600 people who die suddenly each year experience none of these - or so mildly that they dismissed them. 

Adam was a classic example.  Apart from one dizzy spell four months previously, he'd always been a fit, sporty boy, and was studying sports sciences at Greenwich Community College, says his mother. 

'He played football and rugby regularly and even worked as a lifeguard at a local swimming pool.  He was rarely ill and there was never any suggestion that he had any heart problems.' 

This is why the charity wants all young people to have their hearts screened.  'An ECG (electrocardiogram) ultrasound scan could detect these abnormalities before they prove fatal,' says Dr Cox. 

The problem is that the NHS is not geared up to offer routine screening to all young people because there are so few cardiologists with expertise in this field, says Dr Sanjay Sharma, consultant cardiologist at King's College Hospital in London. 

'Cardiac screening of fit and healthy young individuals and athletes is a specialist area.  It is essential that screenings must be overseen by an expert.' 

And so CRY has stepped into the breach - as well as providing grants to train cardiologists to work in the NHS as experts in conditions that cause sudden adult death syndrome, it also runs mobile screening programmes. 

CRY, with the help of Philips, which has donated state-of-the-art equipment, has just completed the first free nationwide screening programme, which ran over two months. 

More than 2,000 people between 14 and 35 were screened.  Thirteen of those were diagnosed with heart abnormalities that could potentially have killed the, but they are now being treated.  Two were identified with Brugada Syndrome, an apparently rare but dangerous inherited condition that interferes with the heart's electrical system. 

'One of the surprising findings was identifying two cases in more than 2,000 people,' says Dr Cox.  'Brugada is usually estimated to affect only one in 10,000 people - it is only through screening that we are getting a better understanding of how prevalent these conditions are.' 

Adam Donnelly was thought to have had Brugada syndrome.  After his death, the Donnelly family were all tested for cardiac problems.  (If there has been a sudden death in the family CRY recommends all first-degree relatives should be referred to their GP to a specialist cardiologist - this screening is available free on the NHS.) 

Adam's sister, Sian, then 12, was found to have no problems, although she will need to be regularly tested until she is 21. 

Adam's dad, Jed, a builder, was also clear.  However, Julie, was a carrier for Brugada Syndrome. 

'You feel very guilty as a mother than you've passed this terrible thing on,' she says.  'But I also feel angry that it's something that can be picked up by a simple test, which is not widely available, and that something can actually be done about it. 

'We are convinced that if Adam had been screened, it would have been picked up.  We wouldn't want anyone else to go through this.' 

Julie has now trained as a bereavement counsellor for CRY - 'in the hope that I can provide comfort for others.' 

The family has also worked hard fundraising for the charity.  In the difficult five years since Adam's death, they have raised £30,000 in his memory to help pay for the heart-testing sessions. 

Jade Moore, who plays for the England Under-19 football team, is one of those who have benefited from such routine screening.  Two years ago Jade, 18, from Worksop in Nottinghamshire, won a Football Association scholarship to train at Loughborough University. 

All scholarship students routinely undergo heart screening.  Jade's revealed two holes in the heart, which had been present from birth and affected the way blood was pumped. 

Her father Adrian, a 46-year-old shop owner, recalls his shock at the diagnosis.  'Jade's always been incredibly healthy and good at every sport.  When she told us she had two holes in the heart and had to have an operation, her stepmother Sarah and I were worried sick. 

'We were told that there are plenty of people with holes in their heart, but it you are an athlete, your risk level goes up because you're pushing your body so hard.  We're just so relieved that she found out and was able to have it fixed.' 

Jade underwent surgery to repair the holes, and within two weeks was back playing football. 

'She will still need to have regular testing, but she has been passed fit and healthy,' says Adrian. 

For the Donnelly's, stories like this make their efforts worthwhile.  'Our lives will never be the same again,' says Julie. 

'What happened to Adam made my question my faith, but we have found a way of going forward and maybe helping other people. 

'If, as part of the screening programme, one young life is saved, then it will have been worth it.' 

For information about having your child tested through CRY's subsidised screening programme, go to www.c-r-y.org.uk; or tel 01737 363 222.