tailslide93: this is what the "RAF Quarterly Review of Aircraft Accidents & Occurrences" had to say:
2nd June 1965 Canberra T4 WT481 39 Squadron
An IRE took off for an instrument flying training sortie with a squadron pilot and navigator. The aircraft took off and climbed to 40,000 ft. Some time later, the aircraft was cleared to descend to 20,000 ft. Approximately twelve minutes after acknowledgement of this clearance, a brief Mayday call was heard and a bearing was taken on the transmission. Search and rescue action revealed that the aircraft had crashed into the sea. From the fragmented nature of the wreckage recovered, the Board of Inquiry found that, in all probability, the aircraft had plunged into the sea at high speed with all crew members aboard. Although the evidence was circumstantial, the most probable cause of the accident was thought to be a loss of control during instrument flying. The weather was excellent at the time of the accident.