Complex sentence: In April 1945, Sir Henry Harwood became Flag Officer Commanding the Orkneys and Shetlands (HMS "Prosperine") until he retired from the service on 15 August 1945 with the rank of Admiral, having been declared medically unfit for further duty.
Split into simpler sentences: In April 1945, Harwood became Flag Officer Commanding the Orkneys and Shetlands (HMS "Proserpine"). He retired on 15 August 1945 with the rank of Admiral, having been declared medically unfit for further duty.