This is part of the reply I received from Sainsbury's Customer Services today (Friday 13th Dec):

"In light of your feedback, I have called the Forest Hill store and spoken with John Parry, Fresh Foods Manager. John acknowledged that the store’s internal heating system was faulty and passed on his apologies for any inconvenience caused. He did confirm that the management team had taken measures to have a temporary heating system put in place until the new year when a new boiler will be installed. The temporary heating system should be in place within 4 working days."