A memorial march is due to be held in Nigeria for a champion of the environment who confronted one of the world's biggest oil companies - and was then hanged.It is the 20th anniversary of the execution of Ken Saro Wiwa, who campaigned against oil pollution in the oil rich Niger Delta by Royal Dutch Shell.He was sentenced to death after being found guilty of involvement in four murders - in a case condemned as a sham and after international appeals for clemency.Al Jazeera's Yvonne Ndege has been to meet his widow in Ogoni - who is proud of his fight against Royal Dutch Shell and for democracy.

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