Preheat the oven to 200ºC, fan 180ºC, gas 6. Cut 600g of the potatoes into chunks and boil for 10-15 minutes until tender. Drain and mash. Mix in the beaten egg and set aside.
Meanwhile, cut the remaining potatoes into 2cm cubes and spread out on a baking tray. Drizzle with 1 tablespoon oil (if using) and toss to coat. Place in the oven and cook for 30-35 minutes until golden.
Meanwhile, flake the tuna into the mash and stir in the sweetcorn. Season with freshly ground black pepper and mix well. With wet hands, form the mixture into 8 evenly sized cakes.
Heat the remaining oil (if using) in a non-stick frying pan and fry the fishcakes for 2-3 minutes each side, until golden. Transfer to a baking tray and cook for a further 10 minutes in the oven, until heated through.
Meanwhile, boil the green beans for 5 minutes. Drain, then serve with the fishcakes and the cubed potatoes.