In August of 1988, my mum found some unexplained bruises and went to the doctor, taking me and my sister Hannah. At seven years old, a few hours at the surgery seemed an eternity, but thinking of that afternoon now, it was sheer blissful ignorance. In an instant, my life was turned on its head. Instead of going home, an ambulance came to take mum to the Royal Marsden hospital in Sutton. At 39, she had been diagnosed with acute myeloid leukaemia. Other than one very special day when we came home from school to find her in her own bed just for the afternoon, she never came out again.

The illness was horrific and her only hope for recovery was a bone marrow transplant. Siblings are most likely to be appropriate donors and, thankfully, her brother, Gary, was a perfect match. Before the transplant, mum underwent intensive chemotherapy. A final blast removed every bit of bone marrow and every last white blood cell in her body, in preparation for receiving Gary's. Unfortunately it destroyed every last beautiful dark brown curl on her head, too. Her immune system was zero and she remained in isolation for two weeks. I remember putting pictures that I had drawn for her through a sterilisation machine so that she could see them.

There is only a 50% chance that any bone marrow transplant will be a success. To reach this point at all, the illness must be severe, and there is a 25% chance that it will return. There is also a 25% chance that the bone marrow will reject the body it goes into or vice versa. In November, Gary's marrow was harvested and transplanted. But the cancer was too aggressive and my mum, Sandra Elms, died on November 15 1988, just three months after the diagnosis.

When my dad told my sister and me, "Mummy has gone to heaven," Hannah (who was 11) began to cry - a sound that came right from the pit of her stomach. It was an animal-like noise that everyone will hear and everyone will make at some point during their lives. Now she says that I was the one making this sound. I think we disagree because we were watching the scenario from afar. It was as if it wasn't really happening to us. Well, these things don't, do they? You don't live in a normal house in the middle of Surrey, as part of a normal family of four one day and then the next, an au pair is picking you up from the school gates because your mum's not around any more, do you?

I have been giving blood since I was 17 and in 2001 at a local session, I saw a leaflet about bone marrow donation. The memories came flooding back and despite the mixed emotions I felt, I knew I wanted to register. I found out later that only 30% of donors are relatives and that making this decision was the best thing I could have done.

I joined the register simply by donating an extra sample of blood for analysis at the British Bone Marrow Registry. In Britain there are just 600,000 people on the BBMR , the Anthony Nolan Trust and the Welsh Bone Marrow Registry. Every year, 24,500 people are diagnosed with a disease that requires a bone marrow transplant.

In April, I received a letter from the Blood Service. Reading it, I was winded by emotion. It told me that another family was going through the same ordeal I had. But I had a chance to help them become a success-rate statistic. This glimmer of hope came at a particularly pertinent time for my family, as my dad and second mum were nursing my grandmother who was suffering from terminal cancer.

I had further tests to get a more detailed view of my tissue type and two weeks later heard that I was potentially a promising match. I was delighted and went for more tests. Four days after my grandmother's funeral, I was told that I was the best match for the patient. There is only a one in 2,000 chance, even though you are on the register, of being asked to donate. I felt privileged to be given the chance to save someone's life.

I had to attend a counselling session and full medical two weeks later, where the procedure was explained to me in full. My bone marrow would be harvested under general anaesthetic. It would require a two-night stay in hospital and a week offwork (for which the BBMR would have reimbursed any loss of earnings had my company not been so understanding). During the hour-long operation, two surgeons would syringe enough marrow necessary for the size and age of the recipient from the back of my hips.

At the medical, I had many more blood tests, a chest x-ray and ECG to ensure I was properly fit. I also met the surgeon who would be carrying out the procedure at King's College hospital in London. Dr Hume Ahmed treats patients in the haematology ward as well as carrying out harvests and so sees the process from all angles. He explained that although there was only a 50% chance of a patient recovering completely, the transplant might let a child live to its sixth birthday, for example, or allow a father to give his daughter away at her wedding. It is easy for us as "well" people, to look on life and death as black and white but in this situation, a few years, months or weeks can make all the difference for a patient and their loved ones.

The operation was set for July 23. I tried to be brave and my husband and I set off the day before. I couldn't help but think of the family on the way to the hospital - I knew how they would be feeling, how nervous they would be and hopeful that this transplant would be a success.

Early next morning, I was taken to theatre and shown the bed that I would be lying face down upon during the procedure. This sight was nerve-racking but I was made a fuss of. (Bone marrow donors are always pampered, I was told by a nurse.) The next thing I knew, I was drifting out of a deep, restful sleep. I was given some local anaesthetic before the general one wore off so my back just felt numb. Although I would never be allowed to meet the patient or learn of his or her progress, the surgeons had taken 650ml (around a pint) of bone marrow, which indicated that the recipient was a child.

After lots of sleep and TLC, I was released from hospital the following day. There was some pain, similar to a bad fall on ice or being kicked by a horse. Standing for too long or walking too fast aggravated the discomfort in my hips and so I had to take things easy, but the main side effect was extreme fatigue. At one point after the surgery, I averaged around 15 hours sleep a day. But the feeling that it had been a privilege to give the marrow far outweighed any pain.

I believe that my mum was a gift of God for the 39 years she was alive and the world is a better place because she was a part of it. She is always close to my mind, and the more I learn about her from my dad, the more she inspires me.

Before the operation, I felt it was a shame that I would never know whether my recipient had made a full recovery - now I am happy not to know. What I have been through over the past few weeks was my gift, just as my mum was to me.

I learned of a shocking twist to my story last week. My uncle Gary, who donated bone marrow to my mum, has been diagnosed with non-Hodgkins lymphoma and chronic lymphatic leukaemia. His one chance of survival is a bone marrow transplant. My mum was the perfect match for Gary. His surviving siblings cannot help and so they are searching the registers for the next best thing.

Maybe it's you.

· If you would like to give blood or register as a bone marrow donor, contact the National Blood Service Helpline - 0845 7711 711, www.blood.co.uk