Wide-eyed with excitement, Christian Velten rested a video camera on boxes before speaking about his forthcoming West African adventure.

The six-minute film - complete with a map of his destination - was to be included in a home-made documentary of his 4,000km solo trek.

The footage has now been released as two Sussex detectives prepare to fly to Mali in a bid to discover the fate of the 28-year-old graduate.

He phoned his family on March 23 last year from Kita in Mali. Since then, there has been no contact.

Christian, from Brighton, planned to spend five months following in the steps of 18th Century explorer Mungo Park on foot, by donkey, camel and dug-out canoe.

He hoped to film his trip and make a documentary on his return to show how the region has changed in the 200 years since Park's ill-fated journey. However, weeks after he made the tape, the Edinburgh University zoology graduate vanished.

In the tape, shown to reporters yesterday, he said: "There is no guarantee there will be no unrest. But this will be the journey of a lifetime. I can't wait to go."

On Monday, Detective Chief Inspector Reg Hooke and Detective Inspector Martin Sapwell will take their forensic, analytical and computer expertise to Africa in a bid to end the mystery.

Christian's retired parents Tim and Pauline, from Burwash, near Heathfield, have already spent a fortune hiring a private investigator to look for their son.

At a Press conference in Lewes, Mrs Velten said: "We've had no word to say anything has happened to Christian and we have to hang on to that. The best scenario is that he has met a nomadic tribe called the Fulani and is travelling with them."

Christian set off on February 7 last year.

Living in Mali without sophisticated communications, his parents did not worry too much initially when the letters and phone calls dried up. However, as the weeks passed without a word, their worry grew. When he did not make his flight home on July 22, their fears grew.

Mungo Park was 34 when he drowned in what is now Nigeria on his second expedition in 1805. Christian's parents began to worry he might have met the same fate or been kidnapped or robbed and left to die.

In May, they contacted the Foreign Office yet four months passed and still no official search had been launched.

Yesterday, his parents criticised the authorities' initial lack of interest but praised Sussex Police for their work now.

The private investigator they hired found there had been sightings of a lone white man travelling in the Mali capital Bamoka and on the Niger between Bamoka and Timbuktu, where part of next week's mission will focus.

Mr Hooke said, with hindsight, he would have liked inquiries to start sooner.

But he added: "We are wholly dependent on the Foreign Office and Interpol setting up effective communications in what is a problematic part of the world."

Christian's parents revealed they had been aided in their search by businessman and adventurer Sir Richard Branson.

He read an article about Christian's disappearance and offered to lend the family a satellite telephone, later used by the private investigator.