Chop the potatoes into small chunks and place into a pan of boiling water. Add a pinch of salt and simmer for 10 minutes, or until tender but not breaking up. Cooking time will depend on the size of the chunks. Drain in a colander, rest over the pan and allow to cool for 1-2 minutes.
While your potatoes are boiling, you can make a start on the salmon. Place the fish (whole fillet, do not break up yet) into a large frying pan and pour over the milk and 300ml water. Bring to the boil, cover and simmer gently for 304 minutes. Remove from the heat and leave to stand covered for 10 minutes. After this time has passed remove the fish from the pan and set on a plate to cool.
Place the spinach in a dry frying pan. Cover and cook gently for 3-4 minutes until wilted. Drain, cool, squeeze out the excess liquid then chop roughly.
Once the potatoes have cooled mash them and beat in the spinach, lemon zest, chives and mayonnaise, and season with salt and pepper.
Drain any liquid from the salmon and flake into large chunks. Place into a large bowl with the potato mixture. Using your hands gently mix the potatoes and fish together. Don't let the fish break up too much. Set aside to cool.
Pour the eggs into a bowl and dust a chopping board with flour. Spread breadcrumbs on a baking sheet. Shape the fish and potato mixture into cakes on top of the floured chopping board. Pat off any excess flour, cover each cake with beaten egg followed by breadcrumbs. Cover and chill for one hour. This will allow the fish cakes to hold their shape when you start cooking them.
Preheat the oven to 110C. Heat the oil in a large frying pan. Fry the fish cakes in batches over a medium heat for 3-5 minutes per batch until golden brown. Keep fried cakes warm in the oven until all cakes are cooked.
Serve with sweet chilli sauce and salad.