Cook the potatoes in a saucepan of salted boiling water for 15 minutes or until tender. Drain well, return to the pan and mash. Leave to cool. Stir in all the remaining ingredients except the polenta and oil.
Put the polenta on a plate, shape the crab mixture into 8 cakes and coat in the polenta. Cover and chill for 20 minutes.
Heat the oil in a large frying pan, add the cakes and fry for 2–3 minutes on each side until golden. Serve with lime wedges and a mixed leaf salad.