When Theresa May became Prime Minister she appointed Fiona Hill and Nick Timothy as her joint Chiefs of Staff. It's a role first imported from the US by Tony Blair, when he brought in Jonathan Powell to help oversee the day-to-day running of government. The position puts Timothy and Hill at the heart of decision-making inside Number 10. So who are they? And how much influence do they really have? Mark Coles reports.

Producer: Ben Crighton.