Set your grill or barbecue to medium heat.
Rub and rinse the new potatoes, then cook them in boiling water for about 10 minutes. Drain, cut them in half and put them back in the pan. Add the butter and dill, toss and put on the lid. Set aside.
Meanwhile, rub the salmon with oil and season with salt and pepper, then grill or barbecue for 3-5 minutes on each side. Blitz half of the watercress with the yogurt, honey, mustard and lemon juice together in a blender. Season to taste, then adjust the lemon juice or honey content to your liking.
Serve the potatoes with the salmon. Place a small handful of watercress on top then drizzle with the sauce.