The Duchess of Cornwall applied a traceable liquid called SmartWater to the Royal Artillery Memorial at Hyde Park Corner. The liquid is used to identify memorial plaques that have been stolen.
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A minute's silence is being observed at midday around Britain to remember the victims of the van attack near Finsbury Park Mosque last week.
Drivers of diesel cars are being hit with an extra £2.45 charge to park in parts of central London.
An inquest is due to open into the deaths of five friends who drowned during a day trip to Camber Sands in East Sussex last summer.