Aha! I believe the answer to be in: http://www.food.gov.uk/foodindustry/guidancenotes/labelregsguidance/foodlotmarkguid

Some food is required to be identified into a 'lot' or 'batch' so if we later discover somone poured rat poison in we can destroy the appropriate items only and not all milk ever made.

These should be of an appropriate size, and if this is larger than the number made on a day, then the best before date functions as this marking. If it's nearly, but not quite, then we append a letter.