A message broadcast over Peking Radio on Monday has allayed anxiety in the Codnor home of Mr. and Mrs. William Cox. The message said that their 23-year old son, Marine Commando William Clive Cox who was taken prisoner by the Communists in Korea, is in good health and being well treated. Marine Cox was reported missing last November, but in January his parents received official notification that he had been captured. Marine Cox, who had previously served in the Royal Engineers, joined the Marines last June alter finding civilian life dull. He was flown out to Korea last autumn in civilian clothes. He is single and was formerly employed by G. C. Ogle and Sons. Ltd., of Ripley."