Peel the potatoes and cut into roughly 3cm/1¼in chunks. Put the potatoes in a large saucepan and cover with cold water. Bring the water to the boil and then reduce the heat slightly and simmer for 12-15 minutes, or until the potatoes are soft but not falling apart.
Drain the potatoes in a colander and return to the pan. Mash the potatoes until smooth or pass through a potato ricer. Season well with sea salt and freshly ground black pepper, then spread out onto a cold plate and leave to cool.
Place the fish fillets in a large saucepan, with the thicker fillets on the bottom of the pan. Pour the milk into the pan, add the bay leaf and season with a little salt and some freshly ground black pepper.
Cover the pan with a tight fitting lid and gently bring to a simmer. Immediately, take the pan off the heat and leave to stand for 10 minutes. This will finish cooking the fish and infuse the milk with flavour.
After the time is up, carefully remove the fish from the milk using a slotted spoon and transfer to a plate. Reserve the milk to make the parsley sauce. Break the fish into large chunks, discarding the skin as you go. Leave the fish to cool completely.
Place the cooled mash in a large bowl and stir in the lemon zest and spring onions. Tip the cooled fish into the mash and use a wooden spoon to mix it into the mash, without breaking up the fish too much.
Divide the mixture into six balls. Flatten each ball until roughly 3cm/1¼in thick and place on a baking tray lined with baking parchment. Place the tray in the freezer for about 20 minutes to firm up. (Alternatively, put in the fridge for 45 minutes.)
To coat the fishcakes, sprinkle the flour for dusting over a plate. Beat the eggs together in a shallow bowl and spread half of the breadcrumbs over a large plate.
Dip one of the fishcakes into the flour, coating well on all sides. Gently pat off any excess flour before dipping the fishcake into the egg, coating all sides. Allow any excess egg to drip off of the fishcake and then place in the breadcrumbs. Turn the fishcake in the crumbs and press them firmly and evenly onto all the sides.
Place the fishcake on a tray lined with baking parchment. Repeat the process with the remaining fishcakes, replacing the used crumbs with the reserved breadcrumbs for the final three fishcakes. (These fishcakes can be chilled in the fridge until you need them but should be used within 24 hours.)
To cook the fishcakes, preheat the oven to 200C/400F/Gas 6. Heat the oil in a large non-stick frying pan. Using a metal spatula, carefully place three of the fishcakes in the pan and fry over a medium high heat for three minutes on each side. Turn carefully to avoid breaking them.
Transfer the first three fishcakes to a baking tray and fry the remaining fishcakes in exactly the same way, adding a little more oil to the pan if necessary. Finish cooking all the fishcakes in the oven on the tray for 8-10 minutes, or until hot throughout.
While the fishcakes are in the oven, make the parsley sauce. Melt the butter in a small non-stick saucepan. Gradually add the flour, stirring with a wooden spoon until it comes away from the sides of the pan. Slowly add the reserved milk, stirring constantly with a wooden spoon to ensure it is lump-free.
Bring the sauce to a simmer and cook for three minutes, or until the sauce is thickened and smooth. Remove the pan from the heat and stir in the parsley. Season with salt and pepper to taste. Divide the fishcakes between six warmed plates, pour the sauce over and serve with fresh lemon wedges for squeezing.