Complex sentence: For most of that time (33 1/3 years) he worked in the BTO's Ringing Unit, before ill health brought about his early "retirement", after which he becoming the Trust's press consultant, regualrly being intervewed on radio and in the press.
Split into simpler sentences: For most of that time (33 1/3 years) he worked in the BTO's Ringing Unit. Ill health brought about his early "retirement", after which he became the Trust's press consultant, regualrly being intervewed on radio and in the press.