He has written an astonishing chronicle of his struggle to cope with her death and his hunt for those who killed her. It tells of late-night internet searches from a South London flat that took him ahead of the Foreign Office and police, bringing exasperation in Whitehall — and threats from the people he believes killed her.

Charlotte, who was 27 when she died, had gone to Rwanda as a Voluntary Service Overseas worker to teach science. Her family had no doubt that she would resume her career in Britain as a microbiologist. Mr Wilson visited her in Rwanda, finding her “happier and more at ease than I’d ever seen her”.

She later e-mailed that “I now seem to be going out with a 6ft 6in Burundian ex-monk” called Richard Ndereyimana, and had become engaged. Mr Wilson “quite looked forward to having one brother-in-law from the richest country in the world and one from the very poorest”.

On December 28, 2000, Charlotte was travelling in the even more dangerous Burundi with her fiancé to meet his family. The bus, the fatefully named Titanic Express, was ambushed. and the couple were killed, along with 19 others.

Mr Wilson, then 25, tormented himself with questions about Charlotte’s death, and about the conflict in Rwanda and the parallel war in Burundi. The attackers were plainly Hutu rebels; they had sought out Tutsis.

He pieced together accounts from survivors, tracked down by aid workers and journalists, and came to believe that the attackers were members of the Forces Nationales de Libération (FNL), a faction linked to those who took part in the Rwandan genocide and led by Agathon Rwasa. The Foreign Office accepted that members of the group were the prime suspects.

The group and Rwasa denied the accusations. But it controlled the territory where the bus was attacked. Western aid workers and analysts on the ground were convinced of its role. After an attack in 2004 African governments declared the FNL a terrorist organisation and Burundi issued an arrest warrant for Rwasa.

Mr Wilson has never established the names of those who ambushed the bus but he feels satisfied that he has worked out who was responsible.

During this hunt, Mr Wilson’s employers, a department store where he was working on a wedding-list computer system, were very tolerant. His mother and other sister finally lost patience. The Foreign Office and police told him that he was interfering in their inquiries. But he is not dismissive of the British Government. “It did help — it made Charlotte’s death impossible for the Burundi Government to ignore,” he says.

He has accepted reluctantly, for the moment, the Burundi Government’s granting of “provisional immunity” to Rwasa as part of an attempt to bring the FNL into a political process. But he is frustrated that Britain accepts this amnesty while saying it is committed to the prosecution of Charlotte’s killers. He hoped, he says, that the immunity would indeed prove “provisional” and that the FNL leaders would still be brought to trial. Asked what kind of justice he would like, he says: “I don’t want revenge. No, I don’t want them killed.”

He now works for a children’s charity, is married and may write a book on forgiveness.

Titanic Express by Richard Wilson is published by Continuum, £16.99

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