Cut the potatoes into large chunks and put in a pan of cold, salted water. Bring to the boil and cook until soft to the centre.
Meanwhile, put the wine, stock cubes and water in a large pan and bring to the boil. Reduce until about a centimetre is left in the bottom of the pan.
Add the cream and simmer, stirring, until well blended. Taste and season if necessary.
Add the fish and simmer for 2-3 minutes. Remove from the heat.
Using a slotted spoon, carefully move the fish into the pie dish, and add enough sauce to just cover the fish, but not enough that the juice is going to boil over in the oven.
Cover the fish with a sprinkling of sweetcorn, a thin layer of leek, then the parsley.
Once the potatoes are cooked, drain and mash, using any remaining sauce from the fish to make smooth. Add a little butter or cream if there's not enough sauce left; it all depends on the size and shape of your cooking dish. Season to taste, then cover the fish with the potato.
Bake in the top half of a pre-heated oven at 220C for about twenty minutes, or until golden.