The Metropolitan Police says it's looking into whether the substance sprayed into the face of an officer may have been ammonia. He was attacked when he stopped a moped rider in Battersea yesterday afternoon.
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His top speed is believed to be the highest ever recorded in the UK by a rider carrying a pillion passenger.
The mayor took time out from City Hall to join Conservative party workers handing out leaflets in Uxbridge and South Ruislip.
No matter where you look, London's population is getting older - with the exception of one of the capital's boroughs.