Cook the diced potatoes in a large pan of boiling water for 12-15 minutes, until tender.
Place the fish in a saute and pour over the milk. Cover, bring to the boil and then remove from the heat and set aside for 5 minutes or, until cooked through.
Drain the diced potatoes well and return to the pan. Add the mint jelly and some salt and pepper and mash together. Remove the fish from the pan and drain it well, discarding the milk. Flake the fish into the potatoes. Add the mushy peas and stir together. Shape and the mixture into four firm, even sized cakes.
Dust the fishcakes lightly with the seasoned flour. Put the beaten egg into a shallow bowl and spread the breadcrumbs on a plate. Then dip the fishcakes into the beaten egg and then into the breadcrumbs.
Heat a little sunflower oil in a frying pan and gently cook the fishcakes for 2 minutes on each side or until crisp and golden. Drain on kitchen paper. The fishcakes may have to be cooked in two batches.
Cut the lemon into wedges. Squeeze a little lemon juice into the mayonnaise. Serve with a dollop of the lemon mayo and a lemon wedge on the side.