Complex sentence: The period between 2001 and 2010 has seen significant change in the UK's foreign-born population, the 2004 and 2007 enlargements of the European Union have lead to mass migration from the likes of Bulgaria, Latvia, Romania, Poland, Slovakia and Lithuania.
Split into simpler sentences: The period between 2001 and 2010 saw significant change in the UK's foreign-born population. In particular, the 2004 and 2007 enlargements of the European Union have lead to mass migration from the likes of Bulgaria, Latvia, Romania, Poland, Slovakia and Lithuania.