The former detective and Crimewatch presenter Jacqui Hames alleged at the Leveson Inquiry there was "collusion between people at the News of the World and people suspected of committing the murder of [private investigator] Daniel Morgan".
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The man was taken early this morning to a south London police station where he remains in custody.
The Chancellor faces tough talks to build a Government while also dealing with the challenge from a resurgent far right.
Watch John McDonnell's speech at Labour Party conference 2017 live as he speaks to Brighton the audience from 12.15pm.