The Dark Heart by Peter Tonkin

Richard and Robin Mariner are thrust into danger as they enter the dark heart of West Africa . . .  - Having helped the President of a powerful West African nation take control of the country, Richard and Robin Mariner are now among a select group of guests at the Presidential Palace. But trouble arises when the dense jungle compound belonging to the President's daughter and her friend is overrun by a terrorist militia led by a charismatic madman, and Richard and Robin are inexorably sucked into the dark heart of the Delta, where the President's grip is weakest, where lawlessness is fiercest, and where they soon come face to face with an uncontrollable evil . . .