Complex sentence: After working for four months in the UK market (October 1998-Jan 1999) she found herself on her first trip to the USA, after a call from the photographer John Mason, who was over in America on an assignment.
Split into simpler sentences: After working for four months in the UK market (October 1998-Jan 1999) she found herself on her first trip to the USA. While in the UK she had received a call from the photographer John Mason, who was over in America on an assignment.