Put the potatoes in a large saucepan and cover with cold water. Bring to the boil and cook for 8-10 minutes until tender. Drain. Return to the pan and mash until all the lumps have gone and you are left with a thick, dry mash. Leave to cool completely.
Meanwhile, heat 1 tbsp of the oil in a frying pan, add the leek and cook for 5 minutes until tender. Remove from the heat, place in a large bowl and leave to cool.
Add the tuna to the cooled leeks along with the cold mashed potatoes, lemon zest, chives, cheese and yogurt. Mix well.
Place in the fridge and leave for 30 minutes – this makes the mixture easier to handle.
Shape the cold mixture into 6-8 round cakes. Sprinkle the flour onto a plate and use it to dust each fishcake.
Heat the remaining oil in a large frying pan and cook the fishcakes over a low heat for 3-4 minutes each side until golden and crunchy on the outside and heated through. You may need to cook them in batches. Serve with the steamed beans.