Heat the oven to 240 C.
Rub olive oil and salt into the potatoes and place in the oven for 1 ½ hours.
Drain the tuna tin of brine. Empty into a bowl and flake. Add sweetcorn and mayonnaise to taste. Add salt and pepper.
Take the potatoes out of the oven and cut a cross into the potatoes and with the thumb and forefinger of each hand squeeze. Place a piece of butter/margarine on top and divide the tuna mixture over the two baked potatoes.
Sprinkle with grated cheese and serve.