Chocs away: The truck that is fuelled by Curly Wurlys and Walnut Whips

John Grimshaw and Andy Pag are off to Timbuktu in a truck fuelled by Curly Wurlys and Walnut Whips. Annie Deakin reports aboard to learn about a surprising new eco-fuel

An early frost has iced the ground where Andy Pag, in a muddy boiler suit, is stomping around in a Somerset farmyard. Bearded and bespectacled, he is securing two 4x4 Toyota Land Cruisers on to a Ford Iveco Cargo lorry. "We're behind schedule, can you give me a hand?" he shouts, chucking a sodden cloth and sponsor stickers for me to paste on to the ageing vehicles.

Just as tractors charge through the neighbouring fields, Pag is firing up his own monster machine, which is set to leave the UK to spend the rest of its driving days in Mali, West Africa.

The extraordinary thing is, that instead of petrol or diesel, Pag, 34, is pumping his truck's tank full of processed walnut whips and Mars bars. It is the latest barmy coup for eco-warriors, some of whom are heralding chocolate as the answer to our green prayers for an environmentally sound fuel. It's enough to get David Cameron salivating like Augustus Gloop, the gluttonous brat in Charlie and the Chocolate Factory. But can "chocolate diesel" make a difference to global warming? Or is it just hot air?

To prove that chocolate fuel works, Pag and his friend John Grimshaw, 39 are driving the first vehicles to run on biodiesel made from chocolate 4,500 miles to Timbuktu. Pag invited me to hitch a ride in his rusty BioTruck for the first leg of their journey. This explains why I am shivering, patent black pumps caked in dirt, and smearing soapy water on to his grimy vehicles.

The expedition expects to be deemed the first carbon-negative trip (as opposed to carbon-neutral) of its kind, as the few carbon emissions produced will be more than offset when the team donate a small biodiesel processing unit to a Malian charity, MFC. The unit converts waste oil products into fuel. "If our calculations are correct," Pag says, "we will save 15 tons of carbon overall." CarbonAided, the climate change organisation, is independently analysing the carbon footprint of the trip from the transport of raw materials to how the biodiesel is manufactured.

While loading the vehicles with sleeping bags, books, motorbikes and a kite, Pag thrusts a sticky jam jar into my hands. "What do you think? Great, huh?" he beams at the thick syrupy liquid slopping about inside. "Take a whiff." Instead of sweet aromas of hot chocolate, the honey-coloured solution reeks of paint. "I'm afraid the exhaust doesn't smell of chocolate either."

Chemically, the fuel is almost the same as normal diesel, except it has fewer carcinogenic pollutants. On board the truck are 2,000 litres of biodiesel, converted from 4,000kg of chocolate.

Weird science meets Willy Wonka at Ecotec, the firm in the south-west of England that makes fuel from renewable resources. Biodiesel is produced by the esterification of energy crops, such as soybeans and oil-seed rape, and modified waste vegetable or animal oils. Otherwise destined for animal feed or waste, discarded chocolate now has a purpose. "If chocolate is dumped in a landfill, it decomposes and produces methane, a worse greenhouse gas than carbon dioxide, which is produced when it is burnt as fuel," says Pag. And so Ecotec's Oompa Loompas are processing rejected chocolates, melted, misshapen or mouldy, into a more precious commodity, energy.

We board the cab and head off to Poole ferry port down narrow country lanes. On the A35, momentum gathers. "We've hit 45mph, now we're talking," grins Pag, increasingly looking the crazed scientist. I wave them off at Poole. From here, the truckers will race through France and Spain, across the dusty length of Morocco, on to the potholed roads of Mauritania, through the Sahara and into Timbuktu. Climate change has affected the settlement. Once a river port, the water is now 20km away and the town's suburbs risk being swallowed up by sand dunes.

So cocoa-derived fuel is the latest eccentric bid for an ethical source of energy. But is the chocolate-powered voyage to the back of beyond anything more than a PR stunt? Pag and Grimshawmet eight years ago burning around the Sahara on motorbikes. Still avid petrol-heads, they have become conscious of leaving a scar on the environment.

Next year, the duo plan to do the first-ever carbon neutral flight across the world's largest canyon, in Kazakhstan.

Following a motorbike accident four months ago, Pag started planning a road trip to Ghana, his wife's home country, for Christmas. "I wanted to get there on the cheap so looked down avenues of getting free petrol. Ultimately, this is a mercenary venture." Perhaps, but it has since snowballed into a global mission to convert gas-guzzlers to alternative fuel.

Pag and John's shoestring budget, meaning all their equipment is recycled, goes hand in hand with their eco-message. "Consumerism is not a very green approach," preaches John. "We won't save the planet by buying new materials. We need to reuse old things." Their vehicles, which were destined to be crushed, were salvaged from scrapyards. Most of the journey will be in the 18-year-old Ford truck that once belonged to an auction firm. This will carry the fuel and the two Land Cruisers which are needed for the final 130 miles when the tarmac runs out. One was bought on eBay and the other, a write-off, was reclaimed from a "bank job".

Grimshaw says, "We are going to drive these vehicles across the Sahara, which proves they still have life in them."

Powering your car with a jerrycan of milk chocolate derivative is not as loopy as it may seem. Rudolf Diesel ran his first engine on pure peanut oil in 1900. "It isn't new science," explains Chris Elvey, director of Ecotec. "The Germans did a lot of work on biofuels during the war because they were short of oil."

Pag has visions of commuters and parents on the school run filling their cars up with chocolate-biodiesel. "It burns cleaner as it contains less pollutants. It also washes the engine because it has better solvents. Most people don't realise that when biodiesel is made correctly, any unmodified diesel engine will run on it, and with the network of producers around the UK growing, it's getting easier for motorists to buy it."

A spokesperson from Greenpeace says: "Some types of biofuels risk pushing up food prices and causing the destruction of tropical rainforests. This is seriously bad news for the environment. Using waste food products such as chocolate or vegetable oil as an alternative to fossil fuels is a great idea. Unfortunately, the volumes are small and there isn't enough to provide for the whole country."

So, with waste resources limited, chocolate fuel will not combat global warming. It is Emission Impossible

Such an optimistic departure was bound to end in tears. By the time the boys had crossed France and entered Spanish territory, their vintage truck had lost second gear and its headlights, had an oil leak and suffered a puncture. "The radio and heater don't work. But hey ho, we didn't sign up for a luxurious trip, did we?," jokes John on the telephone, recalling how the French customs accused them of being terrorists. "Still it's better than being at work as far as I am concerned."

The most dramatic moment so far came when they were driving through the Pyrenees. "Ours is a summer fuel designed for the Sahara," explains Pag. "It doesn't contain any solvents. But it has been so cold, going below -5C in the mountains, that the chocolate fuel was freezing in the filter. It became like a waxy, Vaseline clog." The answer was to add 5 per cent petrol to the solution, which lowers the freezing point. This additional fossil fuel will be factored in with the final carbon calculations.

After arriving in Tangiers last Friday, they are still parked in the ferry port, only 100m from the gate, battling with customs. Pag says, "The regulatory controls around biodiesel are very confusing. It is simultaneously a waste product, foodstuff, oil and fuel so we had to go from one office to the other trying to explain it. The trip has been slow-going but we hope to drive on soon."

How the fuel is made

For Heath Robinson-style chemistry buy one of Ecotec's home biofuel-processing units.

To make biodiesel, you need two basic ingredients oil and ethanol. Chocolate can be mixed with used oil from local chip shops and restaurants. Once the cocoa butter is extracted from the waste chocolate, it leaves sugar which can be used to create alcohol.

Then the cocoa butter is mixed with methanol and caustic soda. Acid is added to neutralise its pH level. Finally, the mixture is filtered to remove soap and contaminants.

The biofuel debate

Biofuel bashing is in fashion at present. The controversy has led to a backlash as confused consumers avoid buying the fuels.

More than 140 UK filling stations supply biodiesel, but it is often actually only 5 per cent biodiesel and 95 per cent fossil fuel; rather like buying a punnet of organic strawberries to discover that most are riddled with pesticides. "Where are the trading standards?" asks Pag. "Garages need a labelling system. Unless that happens, biodiesel will be branded as a false solution.

"Quite simply, people don't understand that there are good and bad biofuels," says Pag. Certain biofuels have a more damaging carbon footprint than their mineral counterparts as crops may need the input of fossil fuels for fertilisers, harvesting, processing and fuel distribution.

Roland Clift, a senior science advisor at the Department for Environment, Food and Rual Affairs, said, "Biodiesel is a complete scam because in the tropics the growing demand is causing forests to be burnt to make way for palm oil and similar crops."

The "good" biofuels (such as "chocolate fuel") rely on recycling techniques and replace the source material's petroleum-derived equivalent, leading to lower emissions.