Sue Williams, 51, wasn't too worried when, one night before Christmas, her super-fit 16-year-old said his heart was beating too fast.  How could she know he would never wake up? 

"Ill never forget the sound of Lisa's trembling voice as I picked up the phone at work. 

"Come home, Mum," she whispered, barely able to speak. 

But before I could even reply, an unfamiliar voice came on the line and explained she was a paramedic. 

"Is it my son?"  I asked, panic rising inside me.  "Then came the words I'd suddenly feared.  "I'm terribly sorry," she replied.  "David has died in his sleep,." 

All I remember now is dropping the phone and screaming. 

How could this have happened?  It was just 10 days before Christmas and David was just 16, with his whole life ahead of him. 

As his mum, it was my job to protect him so why hadn't I been there when he needed me most? 

Strangely, I thought of the Little Britain characters I'd bought for David sat wrapped up under the tree.  And how he would never get to see them. 

I'd been thrilled when David was born, the youngest of my two children.  Having a boy and a girl felt like a dream come true and with just a two-year age gap, Lisa doted on her little brother. 

"Don't worry mum, I'll look after him," Lisa told me as I waved him off on his first day at school. 

A year later David joined a watersports club nearby.  He spent hours there during the summer and I loved to watch from the lakeside as he performed tricky manoeuvres on his wakeboard. 

As the years passed he became more and more active, always on the lookout for the next adventure. 

The bond between us grew stronger too.  Even during the times I was cross at him, he had a way of making me forget why I was angry - he had a glint in his eye which somehow made everything all right. 

When he was 15, he developed a passion for kickboxing.  He trained once a week in the beginning, but that doubled during the few weeks leading up to his death. 

He'd been doing what he loved the night before all our lives were thrown into turmoil.  And later, while  we waited desperately for answers as to why David had died so suddenly, I replayed our last moments over in my mind. 

"My heart's racing, Mum," David complained, just after he'd got home from kickboxing the previous night.  He felt hot to touch, but I presumed he'd overdone the training. 

He had a shower shortly afterwards and headed off to bed at 11pm. 

I remember drifting off to sleep when I heard David come back into the bedroom.  "My heart's still racing, Mum," he told me. 

I asked if he was nervous about a kickboxing event which was coming up in a few days time. 

David shrugged it off, but looking back, I should have seen the warning signs. 

"Night, Mum," he smiled, heading back to bed.  I never saw him alive again. 

The next morning had been like any ordinary day.  I dressed for work, left the kids in bed and asked Lisa to wake David before she went Christmas shopping with her boyfriend Rob. 

Lisa's phone call tore my world apart.  A colleague drove me home and the house looked like a crime scene. 

"I tried to save him Mum, I tried", Lisa wept hysterically.  My husband's face told me everything I needed to know.  It felt as though I'd walked into some unimaginable nightmare as I stumbled through the front door. 

I longed to run into his room and shake him from his sleep, anything to put an end to this awful dream.  It wasn't until two hours later that Stephen and I were finally allowed to see our son.  He looked so peaceful wrapped in his blue quilt, the smell of shampoo still lingering from his shower the night before. 

It felt surreal as I held him in my arms, holding on so tightly I thought I'd never let go.  As David's body was taken from the house, I had so many questions.  Lisa explained how she'd gone into his room and had been unable to wake him. 

After calling 999, she'd given David mouth-to-mouth but it was too late. 

I can't begin to imagine the terror she went through. 

We had to wait weeks for the results of a post mortem before we could piece together what happened to David. 

His death had left a gaping hole in our lives and Christmas Day passed in a haze.  Gifts we'd already wrapped for him lay untouched so Lisa decided we'd each open one and place them in his room. 

David was a huge fan of Little Britain and he loved the fact that David Walliams' real name was Williams, the same as his, so we bought him memorabilia from the show. 

Four days after Christmas, around 200 people came to say their goodbyes at David's funeral at Mortlake Crematorium in Surrey. 

It was heartbreaking to see so many young people, many of them crying. 

The didn't deserve to go through this. 

Saying goodbye to David was the hardest thing I've ever had to do. 

Lisa's bravery was incredible as she read her eulogy through tears.  She talked fondly about David's wicked sense of humour and his love of the water. 

Several weeks after his death an inquest delivered a verdict of natural causes, adding sudden cardiac arrest as a possibility. 

Although the post mortem revealed a slight thickening of the left ventricle in David's heart, that along wouldn't have been life threatening. 

So now, two years on, nobody can say for certain what happened the night he went to sleep and didn't wake up.  the not-knowing is the hardest part. 

The house is so silent now that David is no longer around. 

Stephen and I have since divorced and during the year following David's death, Lisa slept in my bed because she was terrified I too might not wake up. 

We've both been tested for hereditary heart weakness and thankfully we're clear. 

The doctor's will continue to monitor Lisa, who is now 20, as a precaution. 

I'd like to see all teenagers undergo routine cardiac screening because I know that might have saved David. 

We're trying to look towards the future but a day doesn't go by without us talking about David. 

His room is just the way he left it and the door is always open. 

Nieces and nephews play there and Lisa and I often spend nights sitting at David's computer or listening to the songs he loved.  It's now our special place. 

Sue has set up a website in memory of David, where people can also make donations to CRY (Cardiac Risk in the Young)  www.justgiving.com/wakeboard4life

£35 TEST COULD HAVE SAVED HIM

At least eight young people (under the age of 35) die suddenly each week.

Professor Greg Whyte, Chairman of CRY, says: "Sudden Cardiac Death Syndrome is as common as meningitis deaths. 

"Anyone with a family history of sudden death should be tested.  When there ARE warning symptoms, they  can be unexplained fainting, palpitations, chest pain, or unusual shortness of breath." 

A simple £35 electrocardiogram (ECG) can save lives by spotting signs of heart problems.  Depending on your condition treatment ranges from drugs to having a device similar to a pace maker fitted.  If you're aged 14 - 35 and want to be tested, visit www.c-r-y.org.uk or call 01737 363 222. 

EVEN FIT KIDS ARE AT RISK

Comedian David Wallaims is a patron of CRY (Cardiac Risk in the Young).

David says: "I've met parents who'd lost 15 and 16-year-olds to this heart problem. 

You'd think someone fit and healthy, running around, must have a healthy heart.  But if there's hereditary heart disease they can be struck down with no warning.  Listening to their stories was very, very upsetting." 

Before David swam the Channel last year  he had an electrocardiogram (ECG) heart check.  "Scientific things confuse me.  I didn't really know what an ECG was," says David, whose heart test was filmed so others can see now simple it is. 

"I hoped he'd find that I wouldn't be able to do the swim.  Unfortunately, he found I was reasonably fit!  So I thought:  'OK, I'm not ill, so that's good."

You can see David having his ECG test on YouTube or at www.c-r-y.org.uk