In a heavy based saucepan add the potatoes, chicken stock, water, 1/2 teaspoon salt, sugar and 1/2 of the vinegar.
Bring to the boil and then turn the heat down to medium-low and cover. Cook for 17 minutes or until the potatoes are cooked but still remain in tact without being mushy. Remove the lid and increase the heat to high. Cook for another 2 minutes to slightly reduce the liquid.
Drain the potatoes in a colander over a bowl to retain the cooking liquid. Put the potatoes aside and discard all but 60ml of the liquid (depending on the potatoes you may need to add more water if you do not have a 60ml remaining).
In a separate bowl, add a 1/4 of the potatoes along with the cooking liquid. Mash up with a fork until a sauce forms (do not worry if it is thick or a little lumpy).
In a cup whisk together the remaining vinegar, mustard and oil. Pour this into the potato mixture and whisk in again.
Add the remaining potatoes, onion, chives, pickles, seasoning to taste and dill. Fold in with a spatula to mix well.