Complex sentence: Cowell was unable to attend the first day of the Manchester auditions and the final day of the London auditions, at which point the main show's presenters Ant & Dec served as guest judges at the Manchester auditions.
Split into simpler sentences: Cowell was unable to attend the first day of the Manchester auditions and the final day of the London auditions. The main show's presenters Ant & Dec served as guest judges at the Manchester auditions.