In 1982, David Constantine dived into a shallow lagoon and broke his neck. When you listen to him relate the story of his life from that day, you can't help but feel that fate has played more than a small part in shaping his fortunes ever since.
Clearly he wouldn't be able to be a farmer any more, so he went to computer college, where he met fellow student Richard Frost. By 1986 he was working for IBM, but soon realised he wasn't all that enamoured with programming.
However, as part of its employee development programme, IBM gave every graduate the opportunity to spend three weeks in another department. Constantine wanted to indulge his love of photography, but the photography unit couldn't accommodate a wheelchair, so he wound up in the design department instead.
It was an epiphany, and in 1988 he enrolled at the Royal College of Art to study for a masters in industrial design. When Lord Snowdon gave his year a brief to design a wheelchair for the third world, Constantine teamed up with Simon Gue, and the design won that year's Frye Memorial Prize.
Designed to be made from materials found locally, their chair was simple, easily adaptable, low-cost, and suited to the often rough terrain of many poor countries.
With the prize money, and now with Richard Frost on board, the team took the design to disability organisations in India and Bangladesh. On a plane in India, they met an American advertising executive who was so taken by their idea that she invited them to the US where she helped them raise initial funding for their first project.
During the Bangladesh visit, they were asked to return to the country and set up a workshop to make the chairs. So, once Constantine and Gue had finished their degrees, and Frost had quit his accountancy job, they raised enough money from US donors to allow them to spend six months in Bangladesh setting it up.
Word soon spread, and they received invitations from two other charities for similar work in Poland and Cambodia. By late 1991, they were convinced there was huge demand for their services, and Motivation became a registered charity in early 1992. Since then, it has run 21 programmes in 17 countries, and its workshops have supplied around 25,000 wheelchairs. The workshops are run by local organisations and, with Motivation's help, become self-sustaining - as people that can afford to pay for chairs do so. For those who can't afford to buy them, Motivation helps set up wheelchair funds locally. USaid is now Motivation's biggest funder, followed by the EC and the Department for International Development. Last year, its income was £1.3m, and for the first time in 13 years it even has some reserves.
Constantine, who won a Beacon Prize last year, believes that you make your own luck, but it's clear he feels they've had more than their fair share. "While it has all been a lot of hard work, Motivation has always had very fortuitous encounters. Of course we've had our downs, but whenever we do, we keep meeting all these amazing people who help us."
He admits that everything that's happened, even his accident, seems "pre-destined", and adds in his disarmingly frank way that he never had any grand vision to change the world. "I had never thought about going into development or charity work. Even when I had been a wheelchair user for eight or nine years, it never occurred to me what would happen to a person who needed a wheelchair in a poorer country."
Nowadays, Motivation has evolved into a broader development agency. "We have moved on from being very technically focused, to look at the whole quality of life for people with mobility disabilities. It's about far more than just supplying a piece of equipment."
As well as providing an appropriate, fitted mobility device, Motivation gives disabled people the relevant skills to enable them to work. It champions their rights by helping local organisations to lobby their governments and change social attitudes, and forms partnerships to improve the capacity of local organisations to ensure they can carry all this out.
The breadth of Motivation's work is best illustrated by its five-year programme in Sri Lanka, funded by USaid. It is working with local prosthetics organisations to improve artificial limbs, and also with a local disability organisation to design a powered tricycle. The charity has set up an employment scheme, giving disabled people job-seeking skills. At its first employers' fair, where employers were invited to meet disabled people, 34 of the 75 disabled attendees got jobs on the day. It has also helped to establish a textile training business called Sunrise Garments, to train disabled people to work in Sri Lanka's garment industry.
In Tanzania, it has set up a Wheelchair Technologists Training Course at a training centre for orthopaedic technology, which trains students from all over Africa. Since the three-year course began in 1999, 14 people have become qualified and returned home to set up or improve their own wheelchair workshops.
You get the feeling Constantine has found his job for life. "We always said we wanted to work ourselves out of a job, so that we didn't need to exist any more. But with 20 million people in the world needing a wheelchair, we've a bit more work to do yet."