The incident happened in January, when members of the public reported seeing oil on the Thet upstream of Thetford. The Environment Agency traced the source of the discharge to the river Wittle and eventually to EE Pilgrim's premises at Moor Farm, Banham.
Officers found a large oil tank which appeared to have leaked into a surface water drain leading to the river. The company said the tank had been vandalised and up to 3,000 litres of diesel had been lost.
Appearing before Thetford magistrates on 13 November, EE Pilgrim pleaded guilty to causing polluting matter controlled waters, contrary to section 85(1) of the Water Resources Act 1991, and was fined £8,000 with costs of £1,339.