Dad's first thought, given the haste and obvious secrecy surrounding this mission, was that there was some military emergency, some reconnaissance that was urgently needed, and that for some reason the U-2 could not be used. But that aircraft was not equipped with cameras - although Canberras were used as flying camera platforms well into the twenty-first century - and, from that height, the human eye is more-or-less useless as a way of spotting anything on the ground.

The mystery man Rex was clearly familiar with modern military aircraft. He also made it clear that Dad was to concentrate on flying the crate while he gave directions over the intercom to the navigator, confirming the directions to set a direct course to over-fly central London, climbing to 48,000 feet and making best possible speed. He also instructed my father to keep a close lookout.

My father was a very experienced pilot, having spent at least thirty years of his life flying various craft around this planet. He also had exceptionally good eyesight. Even in later life, well into his sixties, he was more able to spot objects in the sky and to provide an instant aircraft identification much more quickly than I could ever manage.

So it was no surprise that it was Dad who first spotted the multi-coloured lights in the sky, flying on what he thought was a roughly parallel course. The laconic instruction from the mysterious American came over the intercom: "head towards the object at eleven o'clock".

At first, my father thought the other aircraft was only a mile or two away, but the true size of the other craft soon became apparent after some minutes flying towards it at 600-plus knots. As Dad described it, it was as large as an ocean-going liner, circular in overall shape and smoothly rounded at the periphery. The bodywork was a deep black, but there were lights streaming from multiple openings or windows all the way around the disk.

It was completely unclear how the strange craft could possibly stay in the air at all. It was making no attempt to get away from the following Canberra. Despite flying at nearly full throttle, Dad reported that he got the strangest sensation that the mysterious flying machine was merely ambling along, deliberately allowing itself to be observed.

Now, it's difficult to see any kind of facial reaction inside a flying helmet and oxygen mask. Looking around at his companions, Dad reported that the navigator's eyes were wide in shock. By contrast, Rex seemed unsurprised but his eyes seemed to have a slightly manic gleam of exultation reflecting the lights from the instrument panel.

The mysterious American had come aboard equipped with several cameras and a powerful torch. As Dad flew in formation with the giant craft, under and over - 'like a tom-tit on a round of beef', as my old man put it - the American shot off reel after reel of film. He also shone the torch through the canopy; they were flying close enough so that the beam of light could clearly been seen passing over the smooth black hull.

After a few minutes, the other craft dimmed its lights to almost nothing, with just an eerie blue glow remaining around some of the orifices which Dad took to be its engines. Rex's twang came over the intercom, breaking into Dad's thoughts.

"OK, I've seen enough. Break off and descend to 45,000. Head west."

Dad complied immediately. Looking behind, he could see that the mysterious craft seemed to darken and then recede into the distance. It was only after a moment that Dad realised that the machine was going straight up. It disappeared after only a few seconds.

There was a instant of strange stillness in the cabin, despite the ever-present roar of the engines. The moment was broken by Rex's voice, instructing my father to perform the strange manoeuvre I described earlier, the significance of which he did not appreciate until he heard about the reports in those 'sensationalist newspapers'.

Why? What was the purpose of the ruse? Dad wasn't sure, but I'm convinced it was what these days we would call 'plausible deniability'. It was a provable matter of record that, yes, a military aircraft was flying over London on that day, on a course which corresponded to any sighting which might have been reported, and which had genuinely been practicing 'unconventional manoeuvres' which might have confused an observer.

In the post-mission de-brief, it was made very clear that the RAF crew were not supposed to tell anyone about this, not now, and not ever. There were appeals to patriotism, which rankled a bit in the presence of so many Americans, and there were vague threats, not least of which was a blunt reminder of the provisions of the Official Secrets Act.

Just at that moment, Mother appeared at the kitchen door to summon us for dinner, effectively terminating the topic of conversation. Dad and I never spoke of the UFO incident again.

Dad had continued his flying career for many years, first with the RAF and more recently with a number of commercial organisations before heart problems detected by the stringent tests that are required of all commercial pilots forced him to retire. Since then, he has lived the quiet life, cultivating his garden and his little circle of cronies, and occasionally acting as a chauffeur for funeral companies.

As far as I can see, his only rebellious act was writing that autobiography, laboriously typing up his stories and anecdotes for what is likely to be, I'm afraid, a frankly miniscule audience. I don't suppose that the book will actually be published now. But I do know that he also vaguely mentioned something about lights in the sky in the same chapter in his autobiography where he reports his antics, although he notes that there was probably a "mundane explanation to this phenomenon". It was probably a huge mistake to write this stuff down at all.

My father died very suddenly, only last week. The funeral is tomorrow. My mother is distraught, inconsolable. I'm pretty upset about it myself, as I'm sure you can imagine. I'll miss him.

In one of her more coherent moments, Mother expressed her surprise at Dad's sudden death. She said that he had remained fit and active, walking the dog twice a day and keeping the kitchen garden in good order. (I remember those runner beans lined up with military precision.) He had been watching his diet after his open-heart surgery, and stimulating his brain by contributing to his Rotary Club meetings, engaging with his circle of friends, and tackling crosswords and puzzle books.

So, despite his age, his death came as a considerable surprise, especially to his GP. I spoke to his doctor while I was helping to tidy up his remaining financial affairs. The quack said to me privately that he could see no reason why he should have passed away, but there had been some subtle but distinct official pressure to avoid an inquest, so he felt he had to enter 'death from natural causes' on the death certificate.

Which leads me to a really important question - does anyone know that he talked to me about the UFO incident? Now I'm looking over my shoulder all the time. Are they out there, coming for me too?

Part 1 Afterword