Place the potatoes into a large pot and cover with water. Bring to the boil over high heat, then reduce heat to medium-low, cover and simmer until tender, about 20 minutes. Drain and allow to steam dry for a minute or two.
In a bowl, whisk together the dry Italian dressing mix, tarragon vinegar, water and olive oil until thoroughly blended. Set aside.
Place the hot potatoes into a large bowl and roughly but thoroughly slice them with a table knife until the potatoes are in chunks. Pour the dressing mixture over the hot potatoes, toss to coat and let the potatoes cool. Add the celery, bacon, pickled cucumbers and spring onions to the potatoes; lightly stir in mayonnaise until all ingredients are well combined and serve.
Dry Italian salad dressing mix can be purchased online.