Mine get - and all of them always have had over the years - a small amount of lard (bacon dripping) each morning. Sunflower oil is useless, you need the animal protein. OK, you are in UK so you can get to any supermarket and buy lard ! Not too much, just a small amount, but regularly. Maybe half a small teaspoonful or even less. Never had the slightest problem with dry skin or dandruff.
Yes, grooming with a hound glove too, but go to Boots The Chemist and buy a tube of Vitapoint. Male hair preparation. Squeeze about half an inch onto your hands and rub them lightly together. Then massage the Vitapoint well into the dog's coat. They LOVE this attention and you will hear him purring ! Leave for 20 minutes or so and then brush out lightly.