Put the salmon fillets in a large pan of barely simmering water with one of the lemon wedges. Cover and simmer for 8-10 minutes until the fish flakes easily.
Meanwhile, defrost the frozen mashed potato in the microwave: put it in a microwavable bowl, cover with cling film and cook on high for 5 minutes, stirring once (do not add any water).
Remove the fish from the pan with a slotted spoon, then flake with a fork onto a plate (discarding the skin). Stir the tartare sauce, flour and parsley into the mashed potato, then fold in the fish. Divide and shape the mixture into 8 fishcakes. Lightly coat each fishcake in breadcrumbs, pressing down gently.
Heat the oil in a large non-stick frying pan. Add the fishcakes and cook for 5 minutes, turning once, until golden and crisp.
Meanwhile, cook the peas in boiling water for 3 minutes. Drain well, return to the pan, add the frozen mint and roughly mash with a fork. Serve with the fishcakes, steamed babycorn and the remaining lemon wedges.