Bring a large saucepan of water to the boil. Add the potatoes, halved or quartered depending on their size, and simmer for 15 minutes until tender. Add the dwarf beans, tops trimmed, for the final 5 minutes.
Meanwhile, place the eggs in a small saucepan and fill with water until they are just covered. Once the water has reached simmering point, boil the eggs for 7 minutes.
Remove the eggs from the pan and run under a cold tap for a further minute to cool (and stop the cooking process). Remove the shells and cut into quarters.
In a small bowl, combine the olive oil, English mustard and vinegar to make a dressing.
Mix the tuna, olives, potatoes and beans together in a large bowl. Pour over the dressing, then add the quartered eggs and gently toss to combine.