Cook the potatoes in salted water until just tender, drain and whilst still hot pass through a ricer. Defrost the peas and mix into the mashed potato with chopped mint, chopped spinach and season. Break the tinned salmon up with a fork and mix into the potato mixture. Form into fishcakes and panure in the breadcrumbs. Cook the fishcakes in vegetable oil until golden and warmed through. For the sauce, simply mix the mayo, capers, dill and lemon. Serve with watercress and a wedge of lemon.