One can never match at home the perfect breadcrumb coating that restaurants achieve. But one can get closer to it than the messy process of applying the crumbs suggests is possible.
For fishcakes, use a similar volume of fish and potato. After boiling the potatoes and draining them, allow them to cool for a while before mashing them. They seem to become starchier - a quality you don't want in ordinary mash but that helps the cakes to cohere.
To cook the fish, I usually put it into a warm gratin dish, pour over a half pint or so of milk (flavoured with bay, salt and peppercorns), cover with foil, and bake at gas mark 6/200C for about 15 minutes. I use the milk to make a bechamel, perhaps with parsley.
Flake the fish into the mash, stir, and form into cakes. Have to hand a plate of flour, a dish of beaten egg, and a plate of breadcrumbs. Coat the fishcakes in flour, dip them in the egg, and turn them in the crumbs. This last process is messy, and sticky; you'll be lucky to be able to achieve a consistent coating.
Heat a generous quantity of sunflower oil, with butter if you like (perhaps to a depth of 0.5cms), in a frying pan, until a breadcrumb sizzles in it. Fry the fishcakes over a low to medium heat until brown on each side and warmed through. You'll find that the cooked coating looks more impressive than the raw one did.