Bring a pan of water to the boil. Add the potatoes and cook for 8-10 minutes until just tender. Drain and return to the pan to keep warm.
For the mackerel, preheat the grill to high.
Put the mackerel fillets on the grill tray, brush with the wholegrain mustard and season with salt and pepper. Grill them for 2-3 minutes, then turn over and grill for a further minute, or until the mackerel is cooked through.
For the dressing and salad, whisk the English mustard, white wine vinegar, rapeseed oil and lemon juice together in a bowl. Season with salt and pepper. Add the lettuce leaves to the bowl and mix to combine.
Divide the mackerel between two plates, along with the potatoes and the salad. Serve straightaway.