Preheat the slow cooker if necessary; see the manufacturer's instructions. Cut each cod loin in half to give 4 portions, then drizzle with the lemon juice and sprinkle with salt and pepper. Add a dill sprig to the top of each portion, then wrap with a slice of smoked salmon.
Put the white leek slices into the base of the slow cooker pot, arrange the fish on top in a single layer, tilting them slightly at an angle, if needed, to make them fit. Add the Noilly Prat and hot stock, then cover and cook on low for 1½–2 hours or until the fish flakes easily when pressed in the centre with a knife.
Lift out the fish with a fish slice and put it on a serving plate, cover with foil and keep hot. Pour the white leeks and cooking juices from the slow cooker pot into a saucepan, add the reserved green leek slices and the capers, if using, and boil rapidly for about 5 minutes or until the liquid is reduced to 4–6 tablespoons. Scoop out and reserve the leeks as soon as the green slices have softened.
Whisk in the butter, a piece at a time until melted, and continue until all the butter has been added and the sauce is smooth and glossy. Return the cooked leeks to the sauce with the chopped herbs and taste and adjust the seasoning. Arrange the fish on plates, then spoon the sauce around. Sprinkle with extra dill, if liked, and serve with baby new potatoes.