A spokesman for Transport for London said the cement problem has been resolved and the Victoria Line is operating a good service.
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Dressed in a hat and dark glasses, the singer and former X Factor judge seemed to travel around the Tube network completely unnoticed.
Ambulance chiefs said anyone with a long-term or chronic illness who feels unwell on Thursday should try to ensure they are not alone.
If you're struggling to stick to your New Year's resolution, this could help kickstart your enthusiasm.