Put the potatoes in a baking tray and bake them in the oven for 45-60 minutes, depending on their size. When they are soft, remove them from the oven and let them cool. You can refrigerate them until you’re ready to make the fishcakes.
Remove the skin of the potatoes, then mash their flesh in a bowl, using a fork. It should be textured.
Cook the corn, cool, then remove the kernels with a knife. (This can be done ahead too. Also, you could BBQ or grill the corn first for a charred flavour).
Drain the tuna through a sieve, pushing the brine out with a spoon.
Add the tartar, tuna, sweetcorn, spring onions, egg and breadcrumbs to the potato. Combine well, then shape the mixture into 4 big or 6 small patties and chill for 15 minutes or so.
Coat each cake with the extra breadcrumbs, then put them on a plate.
Heat some oil in a pan and fry the fishcakes for 2-3 mins on each side until golden.
Serve warm or at room temperature.