Complex sentence: This area covers nearly all of the historic county of Middlesex, and includes the City of London in which lies its cathedral, St Paul's as well as encompassing Spelthorne, formerly in Middlesex but now part of Surrey.
Split into simpler sentences: This area covers nearly all of the historic county of Middlesex. It includes the City of London in which lies its cathedral, St Paul's, and also encompasses Spelthorne that was formerly in Middlesex but is now part of Surrey.