Make the mashed potatoes: Cook the potatoes in a pan of boiling water for 15-20 minutes or until tender. Drain thoroughly, then return to the pan with the butter and mash until very smooth.
Add the flavourings: Add the horseradish, parsley, spring onions and sweetcorn to the hot mashed potato and stir until thoroughly mixed, then add the tuna and season to taste. Allow the mixture to cool slightly.
Make the fishcakes: Shape the mixture into eight thick flat cakes, each measuring about 7cm in diameter. Dust them on both sides with the flour.
Cook the fishcakes: Heat the oil in a large frying pan, preferably non-stick.
Add the fishcakes and cook over a medium heat for 3-4 minutes on each side or until well browned, turning them carefully with a fish slice. Serve hot with mayonnaise and lemon wedges and chips.