LONDON, Nov 8 (Reuters) - Martin Sorrell's S4 Capital reduced its core earnings margin guidance again on Thursday after its net revenue fell sharply in the third quarter, reflecting lower activity in content and digital media.

The ad group cuts its full-year operational core earnings margin guidance to 10-11%, down from 12-13.5% given in September.

(Reporting by Paul Sandle; Editing by Kate Holton)