The outbreak has been linked by the Health Protection Agency to people handling unwashed leeks and vegetables in their homes, which explained why it became so difficult to detect.
The Food Standards Agency's chief scientist Dr Andrew Wadge repeated advice about the need for good hygiene practice by everyone handling food to avoid cross contamination.
Food Manufacture will be holding a Product Recall conference in association with the Institute of Food Science & Technology on November 23. Visit www.foodmanevents.co.uk.