Boil the potatoes until tender, drain and mash with the butter. Allow to cool enough to handle.
Remove the skin and bones from the mackerel and break the flesh into large chunks. Combine with the potato, lemon juice, zest, dill, mustard, salt and pepper. Mix well.
With wet hands form the mixture into eight cakes. Lightly dust with flour, place on a plate or tray and cover with clingfilm before refrigerating for 30 minutes to firm up (this makes the cakes easier to handle).
Meanwhile gently combine sour cream with the rest of the ingredients. Set aside and keep in the fridge.
Heat half the butter and oil and fry the cakes over a medium heat, four at a time, on both sides for 6 minutes on the first side, then about four on the other. Don't turn the cakes over until they have a nice crust. Keep warm.
Serve with the horseradish and apple sauce and lemon wedges.