Our Olympics Correspondent Simon Harris is tweeting from a briefing on the London 2012 Opening Ceremony.
Artistic director Danny Boyle (who directed the Oscar-winning film Slumdog Millionaire) is describing his vision for the show.
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Corbyn was unveiling a poster less than a mile away as Khan gave his final campaign speech to party supporters in London.
Hay fever is an allergic reaction to airbourne pollen. There are 2 million sufferers in London (that's one in 5 people).
The man suffered head injuries after falling from Westminster Bridge and landing on concrete steps.