HIT THE HIGHWAY ON THE ROAD TO HELL

Of the numerous colonists to have descended upon Bolivia down the centuries, most have come in search of the precious metal for which the country is famed: silver. I, on the other hand, have come in search of a commodity whose value is less obvious - a 31-year-old railway engineer called Brian.

Bolivia is the final leg of a journey that has brought Brian (surname Reynolds) from his native Chesterfield to remotest South America by way of Thailand, Laos and Brazil. One of 15 male and three female competitors from countries as diverse as Russia and Costa Rica, Reynolds is the UK's sole representative in one of the world's best- known multi-sports races, which has off-road driving at its core. The Land Rover G4 Challenge was formerly known as the Camel Trophy, in the days when men wore beige and smoked 40 a day. Winning carries serious clout among adrenalin junkies, and comes with the added garland of a brand-new Range Rover Sport.

At its heart is a modern adventure race, held in four countries in four week-long stages. Competitors race off-road vehicles through some of the harshest terrain on earth on the way to a series of gruelling physical challenges. These include running, biking, canyoning, horseriding and kayaking, with the added handicap that the nature of each challenge is revealed only at the last minute.

Reynolds has come a long way since the UK's national selection event, held on a freezing November day in Solihull. I find him in the Bolivian town of Sucre, which lies at a dizzying altitude of 2,790 metres. Filling the main square is a mud-spattered fleet of orange Range Rovers, each being hastily packed with mountain bikes, kayaks, water, food and fuel as the racers, along with a 60-strong support and media team, prepare to head for the nearby hills.

Before the race convoy heads off, I discuss the effects of altitude with Dr Mike Irani, the G4's medical supervisor. "The first symptom of altitude sickness is a feeling of fatigue and lethargy," he says. "After dealing with heat, mosquitoes and dodgy tums, the most serious accident to befall a competitor so far has been a fall from a hotel balcony, resulting in some fractured ribs."

At the start of the six-day Bolivian stage Reynolds is lying in seventh place. Some challenges are individual, but some are undertaken in pairs, and he is partnered with Marco Martinuzzi, a lean Italian lifeguard and ski patrolman. Reynolds is still in contention for the Challenge Final, the last endurance event, in which the top four competitors will decide the overall winner, so every point is crucial.

Ahead of the day's action, Reynolds expresses concern at competing so far above sea level. "In Laos I was worried about the heat and now I'm concerned about the altitude. I am keeping fully hydrated, drinking and weeing loads, but nevertheless I feel quite tired."

The convoy leaves town and, using GPS, we plunge off the sealed road and along a dry riverbed, winding through the shade of willow and cypress trees.

The distances covered by the race are huge. The first day alone covers 350km of the 1,200km total. Along the route is Potosi, at 3,977m the highest city in the world, and the 4,824m Cerro Rico (Rich Mountain), a soaring monument to the area's silver-mining history, which dates back to the Spanish conquest in the sixteenth century. The race convoy's ascent of the mountain proves a brutal learning curve for the driver of one vehicle, as it rolls three times down a short, scrubby hillside. The three occupants are shaken but unhurt, though had the car rolled one more time it would have plunged 300 feet into a ravine.

With the mountains behind it, the convoy crosses the Salar de Uyuni, not only the world's highest dried salt lake (3,650m) but also the largest (10,000 sq km). Travelling at 80mph is highly alarming but necessary, as the G4 Challenge requires racers to drive between each endurance event in a set time. The event in this case involves six kilometres of mountain biking and running in the lake's searing whiteness, and sees victories for Alina McMaster of Australia and Nora Audra (Costa Rica).

Camp that evening is a sea of orange dome tents. With the daytime temperatures of 20C dropping to a numbing -16C, figures huddle round fires as they prepare boil-in-the-bag porridge you could build houses with.

The next morning, the altitude begins to take its toll: a Land Rover employee is flown out by helicopter ambulance with suspected altitude sickness; driving through a village at 3,900m, the entrant from the Netherlands blacks out and is given oxygen and an ECG scan at the roadside.

The cars are also being pushed to their limits. "In Laos, the wet made it difficult to get traction," explains Simon Day, the event coordinator. "Here, the dryness is hard on the suspension."

As we wind through tiny villages and riverbeds, aided by military maps, the landscape grows more arid and llamas with bright pink eartags graze on wide floodplains. Despite their best efforts on the drive to the final event before the Challenge Final Reynolds and Marco miss the cut-off by a minute, thus ending Reynolds' dreams of a top-four finish.