The biggest reason the events of that day would be burnt on my mind, was the reason I was there in the first place, and the one thing I had thought about every day for a year and a half, sometimes constantly and always last thing before I tried to go to sleep at night. My sister Jane had gone to bed one Friday evening and never woken up the following morning. Only 35 and apparently very fit and healthy, Jane had simply died in her sleep.

It made no sense, and still, 18 months on, we do not yet know for sure exactly what happened. As a family, we now know a lot about these conditions, but that Saturday morning in November 2003, nothing had prepared us for what was happening.

It took a long time to get close to some kind of explanation as to why she had suddenly died, as each test ruled out another possibility but shed no light on the probable cause. We began to hear about Sudden Death Syndrome, but with the post mortem giving no real cause and the coroner inexcusably offering an impossible trigger for her sudden death, it would take months of testing of all the closest family members to point towards a cardiomyopathy as the reason for her sudden death. As we found out more, her years of occasional blackouts fitted a pattern.

While we were finding out all about Sudden Death Syndrome, we soon came across the charity Cardiac Risk in the Young, who provided a lot of help and support to us as a family, particularly at the bereavement support day in London, where we met other families who knew how we felt better than anyone else could. By this time, Katie had found out that CRY had a team of runners with guaranteed entry to the London Marathon if they pledged to raise enough money. Once she decided to enter, I knew I had to do it too, to honour Jane and help raise money and awareness for CRY, to try to prevent other families from going through what we had.

Unlike Katie, who was five years younger and a lot fitter, I had a lot of work to do to get in shape. I started with a self-imposed alcohol and caffeine ban and a concerted effort to cycle everywhere. I found a 24-week training plan for beginners, starting slowly and soon picked up the pace until we ran a 10K race on Boxing Day. With the first anniversary of Jane’s death coming up at the beginning of November, October had been a tough month. But as November started and so did the training, it became a focus for my grief and gave me time to think while running. By trying to channel my attention into something that could help others, it was a means of coping.

I started fundraising early on, as I was determined to get the maximum out of this effort for the charity. My first tip to fundraisers, would be to make sure rich and generous friends start off any new sponsor form - particularly if they are competitive enough to try to outbid each other. I had raised over £300 in my first afternoon. I could then see people given that same sponsor form later who maybe thought they might give £5 but, on seeing amounts over £50, decided to stretch to £20. Also, make sure you start any new group of potential donors with the one you expect to be the most generous. Everyone else will match their total.

I also knew that I wanted to get the maximum exposure possible, partly to attract even more sponsors but also to take the opportunity to tell as many people as I could about these heart conditions, to try to prevent further tragedies. I wrote to various local media, press and television, giving all the relevant information, and highlighting possible angles to the story. By an extraordinary coincidence, my wife was expecting our second child on the day of the London Marathon itself.  This gave a positive angle to the story and a way into the much more serious issues. Just before Christmas, our local ITV channel, Anglia, got in touch to ask if they could do the story. They filmed and interviewed Katie, Edwin and me and the story, a full three-minute feature, went out on every news bulletin one day in January.

This then led to other things. The next day at my son’s nursery (kidsunlimited in Fulbourn), a lot of the staff said they had seen the TV news and wanted to help. They soon thought of putting on a mini marathon for toddlers and pre-schoolers at the nursery to raise awareness, and hopefully money, with the parents. This would later lead to even bigger things.

Meanwhile, the training was intensifying, with some longer runs in January up to 16 miles. It was about then that I heard from CRY about the chance to run the marathon in their large foam heart costumes. As I had decided to try to get maximum exposure, I had to do this, even if it meant an added burden for my first marathon. Steve Cox at CRY assured me I could give the costume a try in training and not be committed to doing the marathon in it if it was all too much. He suggested I try runs at 5am to avoid being seen. I opted for the opposite, picking busy times to be seen by as many people as possible. I knew exactly why I was running in it and wanted as many people to spot me as possible. I also put my fundraising web address in large letters on the front and back, to publicise it.

My first race in the heart costume was the Sussex Beacon Half Marathon in Brighton in February. I ran with Katie and we stuck together all the way (except around the 10-mile mark into the wind on the cliff tops, where the heart caught the wind like an airbrake and trying to sprint got me barely above walking pace). We crossed the line together in 1 hour 56 minutes, which surprised us both. Determined as ever to publicise our run, I had sought out the local radio station that was covering the race and my interview was on Juice FM the next day. Katie had also got in touch with her local paper, the Daily Echo in Bournemouth, and had a full page spread, including her web address, which led to some large donations.

Running in the costume was not too bad. It was awkward at first because your arms are pushed out at funny angles by the foam, making it hard to swing them as you normally do when running, but I soon developed an upper-body waddle that did the trick. I was sure the marathon would be no problem. In fact, it took some of the pressure off because if I was going slowly, I could blame the costume but if I was running without it, I would have felt the pressure to run faster than I could cope with.

I wrote to lots of different companies to ask for sponsorship. Almost all wrote back to say they didn’t sponsor individuals but did already donate to chosen charities. One such letter though did bring a four-figure donation from a company that didn’t want to be named because they don’t normally do that. It was only a family link that persuaded them to find the funds on this occasion.

By this time Katie and I had far exceeded our minimum pledge level and had thousands of pounds, and expectation, riding on us. That was when the long training miles and an excessively fast second half of the Flora London Half Marathon around Silverstone combined to give me severe shin splints. With only a month to go, I began intensive physiotherapy, with laser treatment, manipulation and ice baths three times a day to try to make sure I got to the start line. It was then that I began to realise just how much this whole process had focused my grief and how devastating it would be to have to pull out. One way or another I had to cover that distance on foot and complete the marathon for Jane.

I had one last avenue to try to publicise our run. I knew that the BBC would feature stories on charity runners during their BBC1 coverage of the London Marathon on the day. I needed to get in touch with BBC Sport and eventually got through the switchboard to the production manager of BBC Grandstand. I told her my story, emphasizing the picture opportunity of the toddlers running the mini marathon and me in the heart costume with them. Before long, it was arranged that champion athlete Colin Jackson would come to Cambridge to film it and interview me.  [Download clip]  This gave it enormous clout and guaranteed the story was sure to be aired on the day. Colin was superb all day and great with the children. All I had to worry about now was the 26 miles ahead of me two days later.

I won’t go into the race itself in too much detail. My experience of the day was probably much like many others, at least until the blistering heat made running in the costume even more of a challenge. My wife, watching at home and fortunately not giving birth (although I was on the mobile all the time to check), even managed to spot Katie and me filing through with thousands of others at the start, and the banner the family had made for us (right), which was in full view on the screen for ages. Katie ran with me for the first 10 miles but we then agreed she should not be held back and run at a comfortable pace for her. The spectators were great and, if ever you needed a lift, you just had to run along the side of the road having high-fives with the crowd.

The one part of the race that I had expected to be emotionally tough was at mile 25 opposite the London Eye. When Jane died, she had been due to go to London that day with Edwin and go on it for the first time. Having worried what seeing this landmark on the day would do to me, it passed uneventfully as I was prepared for it.

Then I turned into Parliament Square. Big Ben struck three. I had nearly finished the race that was the culmination of a year’s focus and six months of hard training, all of which had given me a channel for coping with Jane’s death. Now it was nearly over, the emotions hit me hard and I ran the last mile in uncontrollable floods of tears for my big sister. Two years older than me, she was the one I had known all my life and living without her now is so hard. The fact that I have now reached 35, the age at which she died, and, like her, have just had my second child in the spring makes this an emotionally tough time. I see all that she is missing out on.

Katie and I both finished fine, slower than I would have wanted but I’ll see about that next time. This time was for Jane and finishing was the most important thing. In the end, the fundraising brought in lots of sponsorship. The mini marathon raised over £1,700. Between Katie and me, we jointly raised over £10,500. The most important thing was that we honoured Jane by doing it and it seemed fitting that we got to tell the whole country about Jane’s death and just how much she means to us. One of the hardest things about Sudden Death Syndrome, unlike many other conditions that cause an early death, is that we never got to tell her all this before she died, at least not in so many words. I hope that in spirit she knows this now.

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Download clip of Philip Allen talking to Colin Jackson - used as part of the BBC's London Marathon coverage

(This video clip is low quality in order to keep it relatively small.  The file is just over 3MB in size, and may take some time to download if you do not have broadband)

If you would like a higher quality copy (in MPEG format) of the full BBC and Anglia news features on Philip Allen (7 minutes in total) please contact the CRY office