CCTV investigations in London Road, Kingston found a mound of fat the size of a bus had reduced the 70 x 48cm sewer to just 5% of its normal capacity.
Thames Water has begun repairs to 20 metres of the damaged pipe and work is expected to take up to six weeks to complete.
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Highs on Thursday of 18C.
Barnet Council ran the school, which was an institution for young people with learning difficulties.
Bowers pleaded not guilty of assaulting his ex-fiancee Sophia Cahill in Croydon in January.