1. Bring a large pan of salted water to the boil, add the potatoes and simmer for 15-20 minutes until tender. Meanwhile, whisk together the mayonnaise, mustard, oil and lemon juice in a large bowl.
2. Drain the potatoes, reserving 2 tbsp cooking liquid and leave to steam for a couple of minutes. Halve the warm potatoes and add to the mayonnaise mixture with the reserved cooking water. Toss to combine then set aside to cool.
3. Once cool, stir through the remaining ingredients and season to taste.