Separate the slices of smoked salmon and arrange them in loose folds on a plate to dry off slightly. Cook the potatoes in boiling, salted water until tender. Drain thoroughly, return them to the pan and mash.
Add the curd or buttermilk, horseradish and salt to taste and beat until smooth. Gradually work in about two-thirds of the flour. When the mixture begins to break up, knead it with your hands, still adding flour, until it leaves the side of the pan clean.
Dust a work surface generously with flour. Turn the dough out and pat out lightly to a round. Turn the round over and, using more flour to prevent sticking, roll out to a round just under 23cm (9in) in diameter. Dust off any surplus flour.
Heat the vegetable oil and butter together in a heavy 23cm (9in) frying pan. Swirl to coat the base and lower sides, then pour any surplus into a cup. Using a fish slice or wide spatula, lift the potato cake into the pan. Cook over a low heat for 5-6 minutes until the underside is browned, then turn it over, returning the reserved oil and butter to the pan. Cook for 5 minutes until browned, then slide onto a flat dish and leave to cool slightly.
Arrange folds of smoked salmon over the cake and drizzle with buttermilk or curd. Sprinkle with pepper and dill and serve in quarters (farls) with a lemon wedge.