I am not a duck or frostbite expert, but it does look like frostbite that has happened some time ago. Chemical damage also would be something to consider. The OP says that it was 6C, but it got colder. Even inside with wet feet, animals get frostbite below freezing, or standing on metal. The most important thing is trying to salvage what is left of her feet once the necrotic (dead) tissue comes off. I would agree with the Epsom salts or salt water soaks daily followed by antiseptic or antibiotic ointment. The salt water would be 4 tsp table salt per gallon of water. Savlon, I believe, is chlorhexidene cream that may be available to use in the UK. Any antibiotic needed would need to come from a vet where the OP is located.