Peel the potatoes, cut into chunks, and cook in boiling salted water for 15 minutes until tender.
Tip the potatoes into a colander and allow them to steam for a few minutes. Then lightly mash the potatoes with a little butter. Season with salt and pepper and set aside.
Meanwhile, place the fish into a medium saucepan. Pour in the milk and bring to the boil. Lower the heat and leave to simmer, very gently, for about 5 minutes, until the fish is just cooked. Turn off the heat but leave the lid on.
Blanch the peas and beans in boiling water and then tip straight into cool water, drain and set to one side.
Roughly chop the herbs and spring onions and set to one side.
In a large mixing bowl flake the fish, add the lightly mashed potatoes, 1 tablespoon of the flour, chopped herbs, a generous pinch of sea salt, black pepper and lemon zest. Mix together well, then add the peas and beans and gently mix them through. You may need a little of the warm milk to loosen the mixture.
Dust a plate with a little more flour. Divide the mixture into 4, then lightly shape and pat into circles, dusting them with flour as you go. Put the fish cakes onto a clean plate also dusted with a little flour then pop them into the fridge for an hour before cooking – this will allow them to firm up slightly.
While the fish cakes set up, make the minted peas.
Blanch the remaining frozen peas in boil water for 1 - 2 minutes, then tip away the water. In a food processor or blender add the peas along with the juice of half a lemon, a drizzle of oilve oil, a pinch of salt and pepper and a bunch of roughly chopped mint. Give it a whizz up together until the peas are just blitzed. Pop the minted peas in the fridge until you are ready to cook the fish cakes.
Once the fish cakes have set, heat 2 - 3 tablespoons of oil in a large frying pan and set over a medium heat. Add the fishcakes and cook for 3 to 4 minutes on each side, or until crisp and golden. You may need to do this in batches.
On a plate, spread over a generous pile of the minted peas, place the fish cakes on top and give everything a generous squeeze of lemon. Serve immediately.