Speaking to ITV News ahead of tonight's switch on in Oxford Street, Jessie J said Christmas helped put London in a good mood - which was what the capital sometimes needed.
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Vlogging from her home in London she has become a YouTube sensation, so successful she has just published her first book.
Health union workers were due to walk out on Thursday in a dispute over pay.
Rail services through Farringdon are being delayed again tonight as engineers carrying out safety checks.