To make the potato salad, put the new potatoes in a saucepan, cover with cold water and season with salt and pepper. Bring to the boil and cook for 15 –20 minutes, or until tender. Drain and allow to cool.
Meanwhile, set up a smoker with the tea leaves and wood chips placed in the bottom. (CAUTION: Tea-smoking generates a lot of smoke. Cook in a well-ventilated area, and open the wok outside if possible.). Put the mackerel fillets on top, cover with the lid and smoke for 2–3 minutes.
In a large bowl, mix together the dill, watercress, spring onions and the cooled potatoes.
To make the dressing, whisk together ingredients in a small bowl.
Pour the dressing over the potato salad and mix well. Break up the fish fillets and stir through the salad. Serve.