Moments earlier the men had ambushed Mr McGrigor's Land Rover expedition, killing a British safari operator, Steve Willis, in a hail of automatic gunfire.
"I gather that shooting people is standard procedure for them, so I was deeply worried," Mr McGrigor told The Daily Telegraph yesterday. "I had injured my leg the previous day, so I couldn't escape."
But instead of killing him, the rebels stole the expedition's equipment, set the Land Rover on fire and vanished into the bush, leaving Mr McGrigor with the blood-soaked body of his friend.
It was a tragic end to an expedition that set out eight weeks ago to travel 4,132 miles from the mouth of the Nile to the source in Lake Victoria.
Mr McGrigor, 44, from Lymington, Hants, was leading the team with Cam McLeay, a New Zealander. Travelling with them in three Zap Cat motorboats were George and Kate Heathcote, also from Lymington, and Garth McIntyre, another New Zealander. A Ugandan soldier was with them to give protection.
They were nearing the end of their journey when the rebels struck in Murchison Falls National Park in northern Uganda, where the Nile dissolves into a boulder-strewn obstacle course of rapids and cataracts. The expedition was charged by an enormous crocodile and cleared 34 sets of rapids.
At that stage Mrs Heathcote left the park and travelled ahead of the expedition, leaving the other members and Mr Willis, a former diplomat who ran the Red Chilli safari camp in the park.
Shortly after dawn on Tuesday the team faced yet another dangerous stretch of water and loaded its boats and equipment on to its accompanying Land Rover, intending to drive around the rapids.
"Two rebels in their twenties in camouflage gear suddenly stepped into the road and strafed the vehicle with Kalashnikov rifles," Mr McGrigor said. "The Land Rover veered off the road and crashed."
Mr Willis, who was driving, was killed and Mr McIntyre was wounded in the head. While the other men escaped into the bush, Mr McGrigor's bad leg prevented him from running and he was captured.
"They forced me to kneel down," Mr McGrigor said. "Then they demanded money. They robbed the vehicle, set it on fire and left in a hurry. My arm and side were burned."
"There was blood all over the vehicle," Mr McGrigor said, acknowledging that their "armed protection" - the soldier Juma Tiri - had been "clearly inadequate".
"I am feeling much better now," Mr McGrigor said in the capital, Kampala. "But I am hugely saddened that Steve Willis, who helped us so much, was shot."
Northern Uganda is the killing ground of the Lord's Resistance Army, whose psychotic leader, Joseph Kony, styles himself a prophet and claims to be fighting to rule Uganda according to the Ten Commandments.
His gunmen have kidnapped 20,000 children for use as soldiers or sex slaves and forced 1.5 million people to flee their villages for squalid refugee camps. They frequently strike in the national park and the Foreign Office has issued warnings against travel to the area.
Fifty international aid agencies, including Oxfam and Save the Children, yesterday demanded United Nations action to bring peace to the region. They said that no UN resolution had been passed on the conflict, which has raged for 19 years and claimed tens of thousands of lives. The rebels have recently been attacking foreigners and Mr Willis was the second Briton to be killed in five days.
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