DID something very strange happen on a windswept Wiltshire down just before midnight at the end of September? Did a top-secret American prototype spyplane crash in the darkness a few miles from Stonehenge, killing its crew?

A group of youthful aviation enthusiasts are convinced they have stumbled on a huge mystery - and claim a cover-up of the "incident" was ordered at the highest level by the British and American governments.

Some of them insist that the wreckage of the secret machine, codenamed Black Manta, was spirited back to the USA after a forced-landing. Whatever happened, there is evidence that the authorities moved very fast to get "something" out of sight of prying eyes.

Spotters aprehended at police road blocks

Plane spotters who picked up distress calls on air-band radios near midnight on September 26 went early the next morning to the barbed wire ringed perimeter of Boscombe Down air base, near Salisbury - the home of British experimental flying. Several spotters were apprehended by local police at road blocks and briskly ordered to leave the area, according to Michael Crutch, 24, the RAF editor of British Aviation Review.

"People were reluctant to talk at first," he said. "They wrote down what they had seen - and we were amazed. One was a serving air force officer who saw an aircraft of unknown type disabled at the end of the runway, covered by tarpaulins. Twin inward-canted tail fins were poking out of the covering." There could be a second mystery aircraft at Boscombe, according to Jamie Hunter, who has been keeping watch on the base at night along with other enthusiasts hoping for a sighting. So far they have been unrewarded. 'sleek, twin-finned aircraft of unknown origin flying over Exmoor' But there have been reports of a "sleek, twin-finned aircraft of unknown origin flying over Exmoor in broad daylight" and loud jet sounds like a "flying train" punctuating the night skies of the West Country. The last was on November 29. "All I know for certain is that a flight plan was filed for a United States Air Force Galaxy transport to fly in and out of Boscombe, a few days after the supposed crash," Mr Hunter said. It was flying back to Palmdale in California where the American Stealth aircraft are built.

Both the Ministry of Defence in London and the Pentagon firmly deny there was ever any incident at Boscombe Down. A spokesman at the base said equipment used to simulate helicopter landings on a warship deck was delivered to the base on the day in question. This was the mystery tarpaulin-wrapped "wreckage". There had been road closures but these were to cover the landing of an RAF Tornado bomber testing a highly-classified anti-missile system which had fouled up in mid-air. But persistent reports from enthusiasts indicate something did happen and that a major operation was launched by the Americans to remove the evidence.

Aircraft callsign did not exist

Air traffic controllers plotted the movement of two civilian jets into Boscombe in the two weeks after the alleged incident. One was a Boeing 737, according to Mr Crutch, but "its registration number, N1178X, does not officially exist". One rumour is that the Boeing removed two bodies, prompting speculation that a cockpit fire killed the crew after their aircraft made a forced landing. The second aircraft was a Gulfstream GIV executive jet, registered to a New York bank.

The description of the twin-finned plane fits advanced US aircraft being built under so-called "black" programmes. The TR-3A Black Manta, a highly-secret reconnaissance aircraft first flown in 1991 and supposedly invisible to radar, is a likely candidate. But why should the US air force be flying aircraft, kept secret from the American Congress, from an airbase in Wiltshire?

There is speculation that the aircraft was a Black Manta possibly returning from a spying mission over Bosnia when something went wrong. But the fact that the reports came from near Boscombe hints that Britain has been allowed the highest level access to American Stealth technology.