In a large mixing bowl thoroughly combine everything except the seasoning, the cayenne pepper and the ingredients for the coating; then taste and season.
(If the fish and potatoes are not freshly cooked and hot, the butter will need to be melted before adding it to the mixture.)
Now cool and chill the mixture for an hour or two to get it nice and firm, then when you’re ready to cook the fishcakes, lightly flour a working surface, turn the fish mixture out on to it and form it into a long roll about 6 cm in diameter.
Cut the roll into 12 round fish cakes. Pat each cake into a neat shape, and dip each one first into beaten egg and then in the dry white breadcrumbs. Now shallow-fry the fishcakes in equal quantities of oil and butter until golden brown on both sides.
Drain on crumpled kitchen paper and serve immediately.