Scrub the potatoes and leave the skins on if they’re new or thin-skinned. Cut any large ones in half so all the pieces are a similar size.
Put them in a large pot, cover with cold salted water, bring to the boil and simmer for 12–15 minutes, until a skewer meets just a little resistance at the centre. Don’t overcook them.
Meanwhile, boil the eggs for 8 minutes, then cool them under cold running water and peel.
Drain the potatoes and leave them in the colander for 2 minutes to steam dry.
Tip the still-hot potatoes into a large bowl, cut into bite-sized chunks, and immediately toss with the vinegar, the Dijon and a good pinch of salt. Leave to cool to room temperature, about 20 minutes.
Mix the mayonnaise and sour cream together. Once the potatoes are properly cool, fold it through gently along with the spring onions, gherkins and most of the herbs.
Roughly chop the eggs and fold them through last, so they stay in pieces.
Season well, scatter with the remaining herbs, and chill for at least half an hour before serving.