And still they come. The Ministry of Defence has just released the latest batch of their UFO files, as part of an ongoing three-year collaboration between the MoD and the National Archives. There are 18 files and while that may not sound particularly impressive, this release – the sixth – runs to over 5,000 pages of documentation. The material comprises sighting reports, letters from the public and papers discussing how to handle the issue when it was raised in parliament – as it occasionally was.

There's an extraordinary claim about Winston Churchill, made by a scientist who wrote to the MoD claiming that his grandfather had been one of Churchill's bodyguards. It's alleged that Churchill met General Dwight D Eisenhower to discuss an incident in the second world war when an RAF aircraft returning from a bombing raid encountered a UFO capable of extraordinary speeds and manoeuvres. It was claimed that they agreed to keep this from the public, to prevent mass panic and to avoid the possibility that people's belief in God and the church would be shattered. MoD officials investigating the claim found no documents to support these allegations, though they admitted that prior to 1967, most UFO files had been destroyed after five years.

Some documents relate to the Joint Intelligence Committee (JIC) – more recently in the news due to their role in producing the so-called "dodgy dossier", which took us to war with Iraq. In 1957 the JIC discussed the UFO mystery at a meeting and concluded that four cases where UFOs had been tracked on radar remained unexplained. The report was delivered to the JIC by the air ministry's head of air intelligence. This sort of high-level interest always gets conspiracy theorists excited, particularly where the word "intelligence" appears anywhere near the phrase "UFO".

Indeed, allegations of cover-ups and conspiracies feature prominently in these files. There are accusations from the public that defence advisory notices were being used to prevent the media running UFO stories. There is correspondence about cases where it's alleged that UFOs crashed (one in Wales, another in the Peak District) and that the wreckage was spirited away by sinister government agents. There is speculation about secret prototype aircraft and even documents about UFO researchers being arrested after trying to break into RAF Rudloe Manor in Wiltshire, where they believed UFO secrets were kept.

The MoD's UFO files occasionally contain documents relating to other mysteries such as ghosts or crop circles. Some newly released documents tell the bizarre story of how, in 1990, a man presented himself at RAF Stanmore and said he'd had a dream about an attack at a military base in London. He felt it was a psychic warning. A few weeks later there was a terrorist bomb attack on the base at Stanmore. The RAF police launched an investigation.

My favourite case is that of the man who placed a bet at 100-1 that proof of alien visitation would be confirmed by the end of the 20th century. He then wrote to the MoD asking for evidence that would support his claim. The MoD gave the standard response, explaining that while the department remained open-minded about the possibilities, they had no such evidence. The man lost his bet.