Cook the potatoes in boiling water until just tender. Drain well. Mash the potato with a small knob of butter. Allow cooling.
Put the fish, spring onions and milk in a saucepan and gently cook until just opaque and cooked through.
Drain the fish and spring onions through a fine sieve. Gently mix with the potatoes, avoiding breaking up the fish too much, along with the sweetcorn, cheddar and a generous grind of black pepper. Form into patties. The cooler the mashed potato is when you do this, the easier it will be to form the patties.
Pour the egg on one plate and scatter flour on the other. Dip the patties in egg and then flour or breadcrumbs and arrange on a sheet of baking paper on a tray. Put the patties in the fridge for at least half an hour to firm up if the patties feel very soft
Heat a large frying pan with olive oil. When the oil is hot, put the fish cakes into the pan. Cook for 5 – 7 minutes or until golden brown underneath and then carefully flip them over. Fry for another 5 – 7 minutes or until golden on the bottom and heated all the way through.