But that flight this week was at the centre of a transatlantic row that saw the prime minister being put on the spot on the floor of the House of Commons and the US secretary of state, Condoleezza Rice, forced on the defensive during a visit to Europe. The Gulfstream V has been identified as having been used by the CIA for "extraordinary renditions" - abducting terror suspects and taking them to secret prisons around the world where they may be tortured.

The recording of flights by spotters like Paul from places as far afield as Bournemouth and Karachi has unintentionally played a significant role in helping journalists and human rights groups expose the scale of the CIA's renditions system. But his impact on such international intrigue largely passes Paul by. "It's not the CIA bit that interests us. You don't even know who owns the plane when you take down the serial number," he said, already distracted as something comes in to land through the grey drizzle. "You keep accurate logs, for your own records."

At the door of the shabby end terrace which houses the Glasgow Airport Aviation Enthusiasts Club, Paul is considering how his hobby got him tangled in such a complex web. "We know now that these planes are run by the CIA, but it's not something we set out to know," he said. "I have seen the planes land in daytime and I've seen them land at nighttime. You never see anyone get off them. Most of the time they are just coming in to refuel, but the ones coming in at night you would expect to see people getting off. But you don't - at least, I never have."

Broadly, planespotters fall into three categories: those who like to take serial numbers, those who like to take photographs, and those who indulge in both.

About 40 miles away, on a mound exposed to wind and rain near the freight terminals of Prestwick airport, Stephen, lugging a lens more commonly used by paparazzi lurking in the undergrowth, is setting up a tripod waiting to see what will land today.

He knows it won't be as exciting as July, when the planes of G8 leaders and their secret service entourages landed at Prestwick, but he's hoping for a good day.

"It's my day off, so I've come over to Prestwick, but I'll go to Glasgow and Edinburgh as well," he said. "I do it mostly for myself - it's been a passion since I was child - but I'll post good photographs on websites too."

Stephen clicks the shutter. He doesn't think this one will make airliners.net, his favourite planespotting website. But he'll add it to the collection of hundreds of other plane photographs.

Despite the particular eccentricity of planespotting - and the obvious capacity for fun-poking - it is not a pastime limited to Britain. In Spain town planner Josep Manchado is part of a small group who gather with their long lenses and foil-wrapped sandwiches at Majorca's Son Sant Joan airport.

In January last year Mr Manchado saw a Boeing 737 on the airport tarmac. He pressed his camera shutter button while speculating idly that some US millionaire was in town. Then he put the picture of the Boeing (tail fin number N313P) on airliners.net, and forgot about it.

Within a few days Mr Manchado starting getting strange calls and emails. They came from the US and from Sweden. "People were asking me questions about the plane. They obviously weren't all planespotters because they were asking questions that people who know about planes don't ask," he said.

Activists and journalists had become interested in the rendition flights. There were also, however, strange calls. "One man wanted to buy up all the photos. He eventually sent me a form in which he asked for everything, including my home address. I didn't give it to him and I never heard from him again," he said.

Months later, he got a call from Germany's ZDF television. A man called Khalid El-Masri had come to them claiming he had been kidnapped by the CIA from Macedonia, bundled onto a plane and taken off to a prison many hours away. Several months later, after allegedly being tortured, he was flown back and dropped in Albania.

One of the planes thought to be involved was one Mr Manchado had photographed. It was believed that it had flown on to Macedonia that very same day. With the photo in their hand, ZDF reporters were able to persuade Skopje flight control to give them a printout of the flight plan. The aircraft had gone from Palma to Skopje and from there to Baghdad and Kabul. Mr El-Masri's story, convincingly told but difficult to believe, fitted.

For those prepared to sift through the endless information complied by planespotters and posted on websites, there are many more clues to the CIA's activities to be found. In Ireland peace campaigners have turned themselves into planespotters.

At Shannon airport Tim Hourigan uses a scanner that allows him to see what air traffic control sees, and he, and other activists, religiously note down the numbers of landing planes. Then, using a combination of Federal Airport Authority Records and planespotting websites, they can track the movements of intelligence planes across the world. "It is a tedious job looking through hundreds of pictures of planes," says Mr Hourigan, who is not a planespotting enthusiast. "But it allows you to confirm and expose the activities of the CIA and our own government."

· The planespotters have been given first names only, as they asked not to be identified.