I have checked my fridge, as I initially thought it must be the problem - the thermometer shows the correct temperature. And it is only meat and poultry from Sainburys that goes off. I don't have the same problem with any other retailer.

The thread on the Sydenham Town forum shows other people are having a similar experience to me. I wonder if there is a problem with whoever does the deliveries to Sainsburys in South East London.