Potatoes in large pan, cover with cold water, add a pinch of salt. Bring to the boil. Simmer for 15 minutes or until tender and drain.
Add the eggs to another pan of simmering water and cook for 6-7 minutes. Drain and set aside to cool in a bowl of cold water.
In a bowl toss together the sliced cucumber and onion with the vinegar, honey and a pinch of salt.
Arrange the watercress and potatoes on a plate, then flake over the mackerel. Lift the onion and cucumber from the pickling juice (reserve the juice) and scatter them over the salad. Stir the yogurt into the pickling juice, then stir in most of the dill to make a dressing. Peel and halve the boiled eggs, then place on top of the salad. Spoon over the dressing and scatter over the remaining dill.
