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London's canals are normally a quiet haven in the centre of the capital, but now a stretch of the longest one, is covered in a thick yellow greasy scum. Experts believe a large section of the Grand Union Canal has been deliberately polluted with dumped cooking oil. It has been declared a category 3 incident by the Environment Agency, which means the clean up operation falls to the charity Canal and Rivers Trust. It estimates that costs will mount up to £10,000 a day. Â鶹Éç London's Transport Correspondent Tom Edwards has the story.