For the wheaten bread, preheat the oven to 200C/390F/Gas 6. Line a baking sheet with baking parchment.
Sift the plain flour, salt and bicarbonate of soda into a large bowl and mix in the wholemeal flour and oat flakes. Mix the honey and treacle into the buttermilk, then quickly and lightly fold the buttermilk and milk into the dry ingredients. Mix the ingredients together using your hands, taking care not to overwork - when the ingredients are heavy, the action should be light.
Divide the dough into quarters with floured hands, then mould each into a round and place on the baking sheet. Dust lightly with plain flour and mark a cross in the top of each loaf with a knife. Bake for 20-30 minutes or until the loaves sound hollow when tapped on the base. Transfer to a wire rack, drape a damp cloth over the loaves and leave to cool. (Wheaten bread should be eaten the same day, or toasted the next. This makes four loaves, but the bread freezes well.) Turn the oven down to 100C/210F/Gas ¼.
While the bread is baking, make the cucumber salad. For the cucumber salad, sprinkle the cucumber slices with salt and leave to stand. Put the wine vinegar in a heavy pan with 100ml/3½fl oz water, the sugar and mustard seeds. Heat gently until the sugar has dissolved, then boil to reduce by half. Remove from the heat. Rinse and drain the cucumber, then mix into the pickling liquid with the dill. Set aside.
Put all the ingredients for the court-bouillon in a large fish kettle with 6 litres/10½ pints cold water. Bring to the boil and simmer for 20 minutes.
Lower the salmon into the court-bouillon. Bring back to the boil, then cover the kettle tightly so no steam can escape. Transfer to the oven and poach for 25 minutes with the oven door ajar.
Meanwhile, make the dressing. For the dressing, melt the butter and keep warm. Lower the duck eggs into a pan of boiling water and simmer for four minutes. Lift the eggs out of the water using a slotted spoon and cool a little under running cold water, then carefully peel off the shells. Mash the eggs in a bowl with a fork, then mix in the capers, herbs and warm melted butter. Season with lemon juice, salt and freshly ground black pepper to taste. Mix well and keep the dressing warm.
Remove the fish from the court-bouillon as soon as the poaching time is up. Cut into 6-8 pieces and serve with the cucumber salad and warm dressing on each plate, with wheaten bread and good-quality butter on the side.