Ellie Harrison explores the countryside around the source of the Thames. She finds out why the source of our longest river isn't always in the same place, before heading up one of the rivers that feeds the Thames, to meet the team that run the UK's only privately-owned palace, Blenheim. John Craven is on the south coast to investigate how inspectors make sure our imported food is up to scratch, and Adam Henson is back on the farm to see how the summer weather has affected his crops and his livestock.