Bring a large saucepan of water to the boil and cook the potatoes for about 15 minutes, until tender. Drain and leave to steam-dry, then mash and transfer to a large mixing bowl to cool slightly.
If you’re serving the lemon mayo with the fishcakes, simply mix the mayonnaise and lemon juice together in a small serving ball, season with salt and freshly ground black pepper, to taste, and set aside until you’ve cooked the fishcakes.
Add the peas, prawns, lemon zest and juice, beaten egg and chives, along with a good pinch of salt and freshly ground black pepper to the potatoes. Mix together to combine well.
Meanwhile, half fill a large frying pan with water and heat until just simmering. Add the salmon fillets and poach for 5 to 6 minutes until the fish is completely opaque. Using a fish slice, transfer the salmon to a plate, pat dry with kitchen roll then flake into large pieces with a fork. Wash the frying pan ready for Step 5.
Gently mix the flaked salmon into the potato mixture, trying not to break up the fish too much. Shape into 8 fishcakes, then roll them in flour, place on a plate or baking tray, cover and chill for 20 minutes.
When ready to cook, remove the fishcakes from the fridge. Heat the large frying pan with the olive oil over a medium high heat and fry the fishcakes, in batches, for 3 to 4 minutes on each side, until golden. Serve 2 per person with salad or vegetables and the lemon mayo (if making).