Grate all the zucchini and put it in a strainer over a bowl. Sprinkle with salt and leave for 30 minutes. Squeeze out the moisture, let sit 10 more minutes and squeeze again; discard the juice. In a bowl, combine the grated zucchini, chopped dill and spring onion. Crumble in the feta into pea-sized pieces, pour in the olive oil and mix with a wooden spoon. In another bowl, whisk the eggs, then stir them into the zucchini mixture. Season with salt and pepper. Gently fold in the flour without overmixing. Heat a generous dollop of sunflower oil in a pan until quite hot. Add a big spoonful of mixture and press into a round fritter about 2cm thick. Fry until crispy outside and soft inside, testing one first and adding more flour if needed. Fry all the mixture and serve warm with tzatziki.