Complex sentence: Although five episodes transmitted in 1974--1975 are still lost, Mornington Crescent makes no appearance before 1978 but was played in every surviving episode of the sixth series.
Split into simpler sentences: Although five episodes transmitted in 1974--1975 are still lost, Mornington Crescent seems to have made no appearance before 1978. It was played in every surviving episode of the sixth series.