Complex sentence: The vast majority of casework is dealt with by the CPS Areas, of which there are 13, each with responsibility for conducting prosecutions in a certain part of England and Wales; each area is led by a Chief Crown Prosecutors.
Split into simpler sentences: The vast majority of casework is dealt with by the CPS Areas. There are 13 Areas, each with responsibility for conducting prosecutions in a certain part of England and Wales, and these are led by Chief Crown Prosecutors.