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It's Britain's top civil servant this week: Sir Mark Sedwill, Cabinet Secretary and the UK鈥檚 National Security Adviser. He's the first person to juggle both roles at the same time. And - what with Brexit, a new PM, perhaps even a looming constitutional crisis - he's doing it at a time of serious uncertainty. So: who is he?

Presenter: Mark Coles
Producer: Ben Crighton