Complex sentence: During development of the Typhoon the design team, under the leadership of Sydney Camm, were already planning design improvements, this process resulting in the Hawker P.1012 (or Typhoon II).
Split into simpler sentences: During development of the Typhoon the design team, under the leadership of Sydney Camm, were already thinking of ways in which the aircraft could be improved. This process resulted in the Hawker P.1012 (or Typhoon II).