Cook the potatoes in simmering salted water until tender but still firm, about 20 minutes.
In the meantime make the dressing. Combine the olive oil, wine vinegar, mustard, sea salt and pepper in a large bowl and whisk well until thickened. Add the capers, spring onions and chopped herbs and toss lightly.
Drain the potatoes well and slice them thickly as soon as you can handle them. Toss carefully in the dressing, making sure the slices dont stick together.
Arrange on warmed plates, drizzle with any extra dressing and serve warm.