There's not much to see in the Nubian Desert except, of course, sand. So when Jason Lewis spotted a Hollywood star ride towards him on a motorbike followed by a cavalcade of cameramen, he thought the desert heat was playing tricks on his mind.

However, the suspected mirage in the shape of Ewan McGregor turned out to be real, and the actor was so impressed with Lewis he handed him £1000.

Lewis, the 39-year-old British adventurer, is on the penultimate section of his muscle-powered round-the-world marathon.

The traveller from Dorset has kayaked, pedaloed, roller-bladed, swum and walked around three-quarters of the globe over the past 13 years.

He bumped into the Star Wars actor, from Crieff, while cycling through the most remote tract of the Nubian Desert in northern Sudan last Monday.

He had passed through the small town of Wadi Halfa and was cycling through the desert in temperatures of more than 45 degrees when he spotted McGregor and Charley Boorman, who are themselves engaged in an epic journey from John O'Groats to the southern tip of Africa.

"The only people I saw on this last stretch from Abri were the British actors Charley Boorman and Ewan McGregor plus production team, en-route south on their latest Long Way Down expedition by motorcycles," said Lewis.

"I don't know who was more surprised to see who: myself to encounter this huge entourage of support vehicles and camera crews in tow, or Charley and Ewan to see this lone bicyclist in a desert region where people never usually travel alone or unsupported because of the extreme heat, lack of water and possibility of attack from wild animals, which the locals assure me are out there.

"We crossed paths on the most remote section of the desert highway where at most you'd expect to see a lorry carrying locals to or from Halfa."

He added: "They too were finding the going tough. Since leaving Halfa the previous day they'd broken two sets of rear spring suspensions, putting one of the bikes out of action.

"When I told Ewan he could expect similar road conditions for the next 600-plus kilometres he pursed his lips and said nothing."

Lewis's trip, which is being monitored by the Guinness Book of Records, is largely self-financed, and he has done much of the fundraising along the way.

He discussed how difficult it was to raise money with the actors, adding: "Even they too have found it excruciatingly difficult to procure sponsorship from the UK.

"When I told then that we'd been trying to get financial backing from the UK for 13 years but without success, I think they felt bad and reached into their emergency fund to share their budget a little.

"Before parting ways the producers made a very generous cash donation of £1000 to the expedition.

"It's ironic that the largest contribution we've received from a single UK source has been another expedition."

Originally Lewis only "came along for the ride", joining his friend Stevie Smith in 1994 to pedal a boat across the Atlantic.

Smith left, but Lewis decided to continue on his own. He has since become involved in a UNESCO project to use video to connect classrooms in 470 schools around the world.

His journey has been delayed by accidents, including almost losing a leg, and contracting malaria. His original schedule was to complete the trip in two-and-a-half years.

McGregor and Boorman's journey, which is being filmed for the BBC, will cover 15,000 miles in three months.

They set off from Scotland last month with only McGregor's father Jim and brother Colin, an RAF pilot, present to wave them off from John O'Groats.

McGregor is a goodwill ambassador for UNICEF and the actors will visit the organisation's projects in Ethiopia, Uganda, Malawi and Zambia.

The Long Way Down will also raise funds for the Scottish children's hospice charity CHAS, of which McGregor is a patron.

While in Malawi they will be joined for a fortnight by McGregor's French wife Eve, who has been taking motorcycling lessons in advance.

McGregor described in his diary how they are now down to only two working bikes because of the pounding that they have been taking on the road.

He said: "I come off all the time; I'm off more than anybody else, when it comes to falling off I am the team faller.

"When they go, they go, and there's nothing you can do about it. So luckily more often than not we drop them when we are just standing still.

"I had a few falls on some sand going through some towns, and you're going down these walled streets with people everywhere, in deep sand, and I hated it.

"I hate riding in sand. It makes the front wheel just wiggle about and you think you're going to fall off. And when you put people and children into the equation it's just terrifying. It's horrible."

Three years ago McGregor and Boorman, son of the film director John, travelled 20,000 miles around the world on their first motorbike journey.

They set off from London in April 2004 and spent three months crossing three continents before ending in New York City's Battery Park.