Place the potatoes in a pan of water, bring to the boil, cover and cook for 12-15 minutes until tender. Drain and leave to steam-dry, then mash. Meanwhile, season the salmon and grill for 5-6 minutes until just cooked. Cool for a few minutes, then break into large flakes. Mix the potato, ketchup, mustard, zest, herbs and some seasoning. Lightly mix in the salmon, taking care not to break it up too much. Shape into 4 large fish cakes. Put the flour, egg and breadcrumbs in 3 shallow dishes. Dip the cakes into the flour, dust off any excess, then dip in the egg, and finally coat in breadcrumbs. Heat the oil in a large pan. Fry the cakes over a medium-low heat for 3-4 minutes each side until deep golden and heated through. Serve with salad and lemon wedges.