Everyone knows the London Marathon raises millions for worthwhile causes, but few know that small charities have to rely on the luck of the draw to place even a single runner.

The reality is that getting people into any challenge event is big business. Running, cycling, walking and swimming for charity is a great way for people to raise money for a cause they believe in and want others to care about. It is the lifeblood of charities, an essential element of fundraising, but there are costs involved that the public are seldom aware of.

A ballot winner for the London Marathon has to get into the first 125,000 entries and then has a one in seven chance of a place at a cost of £35. The runner can then choose the cause that he or she wants to run for from those listed that have paid for advertising.

Charities that want to buy places have a different entry point. World Child Cancer has a Silver Bond place, which means we get one spot every five years at a cost of £300. The only way to improve on this is to purchase an advertising package with one of the companies that own the remaining available places; these retail at about £2,500 in exchange for each runner. Small charities like us, with no advertising budget, simply don’t get off the starting block.

As a social entrepreneur, I support the profitability of events. When you participate in a well-organised event, you can see for yourself the costs involved. The chaos caused by lack of water at the recently aborted Sheffield half marathon is rare. For the most part, events are impressively thought through with cheer points for supporters, water and food in ample supply and a well marked-out route supervised by trained stewards who ensure the participants and supporters remain at a safe distance. But the reality is that the bigger the event, the bigger the charity because available advertising leads to packages that fall beyond the reach of small charities.

Even if charities can buy places, they still have to hope participants fundraise successfully. Most do a great job but it doesn’t always work out. Last year, three city boys did the London Triathlon at a cost to World Child Cancer of £185 for each place. They raised nothing, leaving us out of pocket and leading to the decision to withdraw from the event in future years. Our funds are too precious to gamble.

Like millions of people around the world, I will be cheering the London Marathon runners on this weekend. My advice to other lucky ballot winners is to choose a small charity. We rely on the public funding us and will never have the budget to advertise our services through official race media. You don’t have to be Sir Steve Redgrave – who raised a staggering £1.8m for his chosen charity in 2006 (though it would be nice) – just someone brave enough to slog it out for 26 miles and curious enough to look beyond big charity advertising to the smaller causes doing a great job.

Allison Ogden-Newton is the chief executive of World Child Cancer.

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