Chocolates and Cyanide

A box of milk chocolates is delivered to a man just before he leaves to fly to work in Botswana. He does not open it and just tucks it into his packed suitcase. His suitcase is delivered in error to a lady passenger in Johannesburg, who helps herself to one of the chocolates and dies of acute cyanide poisoning!
A concerted effort by the police in South Africa and England have to determine who was the intended victim, and who placed the cyanide in the chocolates, but the identities keep them mystified until they find who purchased the chocolates, and who stole the cyanide. The activities of a string quartette throws up several red herrings…