Rub the peppers with 1 tablespoon oil and cook under a preheated hot grill for 15 minutes, turning often, until charred all over. Put in a plastic bag and leave to cool a little, then peel away the skin and discard the seeds and cores.
Mix together the sweetcorn, spring onions, egg and 2 tablespoons mayonnaise. Carefully stir in the crab meat and season to taste. Form the mixture into small cakes using your hands. Scatter the breadcrumbs and polenta over a plate, and dip each cake into the crumbs until well coated. Transfer to a plate and leave to firm in the freezer for 5 minutes.
Whizz together the roasted peppers and remaining mayonnaise in a food processor until smooth and add chipotle sauce to taste. Heat the remaining oil in a large, nonstick frying pan and cook the cakes for 2–3 minutes on each side until crisp and golden. Serve with the red pepper mayonnaise.