Why is MI6 so open about the threat of terrorism?

The new head of MI6, Alex Younger, has taken the unusual move of making a public speech warning about the "unprecedented threat" posed by terrorism to the UK.

Richard Barrett, who used to run counter-terrorism operations for the UK's Secret Intelligence Service, of MI6, says it's part of a deliberate strategy by the service to reassure the public of their legitimacy.

The intelligence and security services are increasingly "at the centre of government" because of continuing terrorist threats, he tells BBC Radio 4's PM programme, "so I think it's reasonable for the public to have a bit of debate about what they do".