He has uncovered new evidence about the final days of Colonel Percy Fawcett, who was last seen in the Amazon rainforest in 1925. Allen retraced the footsteps of his predecessor with only a camcorder for company and the story of his discovery, The Bones of Colonel Fawcett, will be broadcast by the BBC later this year.
Colonel Fawcett disappeared with his son and a companion after crossing a 1,500-mile stretch of jungle in search of the Lost City of Gold in Mato Grosso, Brazil. When the letters that he wrote home describing his journey stopped, a search was launched amid rumours that he had been killed by Calapalo Indians.
In 1927, a US Navy commander found Indians wearing a nameplate from one of the colonel's cases as an ornament, but 16 further expeditions failed to discover his fate, although it is claimed that his bones are housed in a museum in Rio de Janeiro. The last attempt, led by a New York banker and a Brazilian businessman in 1996, was aborted after 12 of the 16-man team were taken hostage by the Calapalo and released in return for Jeeps and boats.
However, Allen claims to have finally uncovered the truth. According to Bob Long, the executive producer of the BBC's Video Diaries series, the breakthrough was down to a combination of strategic bargaining and the unthreatening nature of Mr Allen's lone quest.
"We knew that the Indians had wanted motors from the previous expedition so we sent out a contact to meet them and asked for their help in return for an outboard engine. They were very specific and requested a Yamaha 80 to power the boat they used to rush medical emergencies down river.
"To our knowledge they have never been filmed before and had had very little contact with Westerners, but they came to trust Benedict because he spent a lot of time with them."
This exchange of technology and trust gave Mr Allen unprecedented access to the Indians and led to the kind of off-beat filming that characterised his earlier adventure diaries.
The perilousness of his 1,000-mile solo walk along Namibia's aptly named Skeleton Coast, broadcast in 1997, was brought home to British audiences when his camels nearly died of thirst and he kept them alive by sharing his own dwindling supply of water.
Later, the diary of his 3,000-mile trek around the rim of Mongolia was given a peculiar twist when he came across a family of isolated nomads too busy too talk to him because they were watching The Fugitive on satellite TV in their tent.
Concerned viewers rang the BBC when three horses and a camel died after being attacked by flies while they rested at a lakeside before the Gobi desert crossing.
Allen's travelogues developed as an offshoot of the Video Diaries project which celebrates its 10th anniversary this year against a backdrop of strong audience figures that demonstrate its unique positioning at the crossover between cult and mainstream viewing.
Its success is also an indication of how the best of the BBC's community programming has transcended its worthy Seventies origins to become the kind of cutting-edge television that, according to some critics, has inspired the plethora of fly-on-the-wall documentaries.
"We ask the diarists to take risks with filming in return for shared editorial control," says Mr Long, who has overseen the project since its inception. "That way you get more intimacy and honesty out of people."
At first, the disdain for so-called "Toytown TV" within the industry was so great that Mr Long found himself interviewing shop workers for assistant producer positions.
This changed within a couple of years when the diaries began to pick up major awards, first with Nick Danziger's film, War, Lives and Videotape, a documentary that followed Danziger's rescue of orphaned Afghan children from a mental institution.
Another forthcoming diary, Giving Up Baby, is considered by Mr Long to be one of the best of the 90-odd programmes that have been made. It is the story of a young pregnant woman who has decided to give her baby away because she feels that the child would have a better life elsewhere.
According to Mr Long, the diary is a powerful depiction of the kind of urban nightmare experienced by the sorts of people effectively disenfranchised in late Nineties Britain.
"The best diaries are always those that feel like drama but that are too good to have been scripted," he said.