London Mayor Boris Johnson Marks The Start Of Building Works For The Northern Line Tube Extension [Getty Images]

LONDON, ENGLAND - NOVEMBER 23: London Mayor Boris Johnson gestures as he poses for a photograph to mark the start of major works to build London Underground's Northern line extension from Kennington to Battersea via Nine Elms at Battersea Park on November 23, 2015 in London, England. The extension will service new developments in and around Battersea Power Station where a studio flat that sold for close to 1 million GBP in the spring is soon to go back on the market with a price tag of up to 1.5 million GBP, despite not having been built yet. (Photo by Carl Court/Getty Images)

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