My caller wishes to remain anonymous - in fact to be kept well out of it. Entirely understandable. Story in a nutshell: customer brought back a packet of crisps (well-known brand) and found a dead mouse in it just minutes after buying it.
The environmental health officer was contacted, duly visited and took the offending packet away to do some forensics on it. Apparently, they would be able to tell where the mouse came from and how long it had been dead.
Except in the end they couldn't. The evidence was inconclusive. The cash & carry was given a clean bill of health and it was deemed possible that the mouse may have got into the packet in the store: it's highly unlikely it got in during the production stage as there's a lot of quality control.
I don't think it's particularly incriminating that the retailer had rodents. They are very common and they breed fast, so you could wind up with an infestation in weeks.
As we all know, the worst thing you can do to a customer is poison them, which arguably could have happened if this customer had consumed many of the contaminated crisps or inadvertently eaten bits of mouse. The statistics are scary. More than five million people suffer from food-borne illnesses each year.
Anyway, the buck stops with the retailer as far as the customer is concerned. If goods are inferior then it is your responsibility under the Sales of Goods Act 1979 to put it right. You might be able to get redress from your supplier but it's very difficult to put the food chain into reverse and send back any rotten eggs, as it were.
Prevention is best. Inspect your incoming goods (I have it on best authority that packaging equals nice nest, des res, in mice terms), and your perimeters for any gaps. I'd like to say get a cat, but I can't. Cats aren't as clean as they're cracked up to be and health inspectors know this.