Boil the potatoes (whole, leaving skin on) for 15 minutes until a sharp knife slides into the centre easily. Drain.
Make a vinaigrette by whisking together the olive oil and vinegar, and seasoning well with salt and black pepper. Cut the potatoes (whilst still warm) lengthways into halves or quarters depending on their size, then toss them through the vinaigrette in a large bowl and set aside to cool.
When the potatoes are completely cool, stir through the mayonnaise and mustard, finely chop the spring onions, capers and gherkins and add all to the bowl and mix.
Finally, chop the parsley and mint and stir through the salad, then top with the cress.