Cook the potatoes in a large saucepan of boiling, salted water until tender; roughly 20 minutes.
Meanwhile, prepare the dressing by whisking together the honey, wholegrain mustard, white wine vinegar and seasoning until smooth. Whisk in the groundnut oil in a slow, steady stream until you have a thickened dressing. Cover and reserve to one side. Drain the potatoes and allow to steam off for a few minutes. When cool enough to handle, slice in half and reserve in a bowl.
Bring another large saucepan of salted water to the boil; cook the green beans for 3-4 minutes. Drain and toss with the potatoes. Add the dressing, toss well to combine, then season the salad. Transfer to a wooden salad bowl and garnish with the mint. Serve hot.