Complex sentence: The Liverpool Urban Area defined by ONS covers Liverpool and its contiguous built-up areas, with a population of 816,216 in the 2001 census which is down 2.6% from the 1991 figure of 837,998.
Split into simpler sentences: The Liverpool Urban Area defined by ONS covers Liverpool and its contiguous built-up areas. According to ONS, it had a population of 816,216 in the 2001 census, down 2.6% from the 1991 figure of 837,998.