Preheat the oven to 200°C.
Place the fish (preferably skinless) in a pan with the leeks and peppercorns and put in enough cold water to just cover.
Place a lid on the pan and bring to the boil then reduce to a simmer and cook for 10 minutes.
If your fish has skin on then remove it at this stage.
Drain the fish and leek mixture reserving the liquor, keep the fish and leeks in lidded pan to keep warm.
Make a roux by melting the butter/marg over a medium heat and then stir in the flour (I like to use a whisk) and cook for a minute stirring constantly.
Combine the milk and fish stock and stir/whisk these in off the heat gradually until you have a smooth thin sauce.
Put back on the heat and stir/whisk constantly until thickened (coats the back of a spoon).
Add the grated cheese and mustard and stir through until melted.
Place half the fish pieces and leeks in a deep ovenproof dish followed by half the sauce, half the breadcrumbs and then repeat, finishing with a sprinkle of cayenne pepper.
Place in the oven for 15 minutes until topping is golden