Prince Harry, the Duchess and Duke of Cambridge with their wands on the set used to depict Diagon Alley in the Harry Potter Films
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The so-called cycle super highway from Westminster to the Tower of London is one of Johnson's flagship transport projects.
Sunday is expected to be the hottest day, with the mercury due to top 26C in London.
London Assembly Tory leader Andrew Boff claimed campaign tactics to undermine Labour candidate Sadiq Khan had damaged community relations.