Peel the potatoes. Place in a pot and cover with 1.5 litres of cold water. Add rosemary and salt. Bring to the boil with the lid closed. When the water boils, cook the potatoes for 20 minutes. Drain and leave to cool.
Cut the potatoes and pickled gherkins into even, fine slices. Put everything in a bowl.
Lightly heat the water for the dressing. Then add all remaining ingredients and stir in. Pour the dressing into the bowl over the potatoes and stir well. Cover with a lid or tin foil and leave to stand for at least 2 hours. The potato salad tastes best the next day.