Lauren Martell claims to have witnessed an alleged incident involving Rolf Harris in the bathroom of a London music studio in the early 1990s.
The woman contacted the Metropolitan Police in November when she first heard of the investigation into Jimmy Savile.
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Run by the NSPCC the helpline allows employees in any sector to raise concerns anonymously.
PM used a speech in Hamburg to say that the UK will not 'pull up the drawbridge and retreat from the world' but Europe must change.
It is set to remain unsettled into Saturday, becoming drier on Sunday.