Grill the bacon until nicely sizzling on both sides. Leave to cool and chop roughly.
To make the dressing, add the mayonnaise, grainy mustard, lemon juice, black pepper, and olive oil in a mixing jug. Stir well to combine.
In a large mixing bowl combine the potatoes, bacon, chopped eggs, and spring onions.
Add the dressing and stir gently until the salad is well-coated.