Boil the potatoes until fork-tender, about 20 minutes depending on size, then drain. Cook three eggs until the whites and yolks are set; cook the fourth to your preferred firmness for the topping. Cool the eggs in cold water and peel.

Cook the beef bacon in a pan until crisp and cooked through, following its package instructions. Chop into bite-sized pieces.

Whisk the yogurt, mayonnaise, mustard, pepper and a little salt. Add the milk gradually until creamy; you may not need all 5 tablespoons.

Roughly mash the drained potatoes and three peeled eggs into the dressing. Fold in the sliced cucumber and chopped bacon. Taste and adjust seasoning.

Halve the reserved egg and place it on top. Serve promptly, or refrigerate the salad until serving and add the topping at the end.