Question:
Which employer did Derman Christopherson work for in Sep, 1964?

Context:
 Derman Christopherson works for Imperial College London from Jan, 1955 to Jan, 1960. 
 Derman Christopherson works for Durham University from Jan, 1960 to Jan, 1979. 
 Derman Christopherson works for University of Leeds from Jan, 1949 to Jan, 1955.

Answer:
Durham University