Peel and cube the potatoes. Cook in a pan of boiling water for 15 mins or until tender. Drain and mash with 4 tbsp olive oil until smooth. Season to taste. Heat the cheese sauce in a pan, stir through the prawns and spinach, and cook gently for 1-2 mins. Spoon the mixture into a medium pie dish and top with the mash. Pop under a hot grill and cook for 6 mins until golden and the filling is bubbling.