Complex sentence: The unit was established in 1933, taking on responsibilities of the Empire Marketing Board Film Unit and was headed by John Grierson, and was set up to produce sponsored documentary films mainly related to the activities of the GPO.
Split into simpler sentences: The unit was established in 1933, taking on responsibilities of the Empire Marketing Board Film Unit. Headed by John Grierson, it was set up to produce sponsored documentary films mainly related to the activities of the GPO.