Preheat the oven to 200C/180C fan/Gas 6. Put a baking tray in the oven to heat.
Make the mashed potatoes and set aside – see Notes.
While the potatoes are boiling you can get on with cooking the fish and making the cheese sauce – to cook the fish, heat a large lidded saucepan on a medium to high heat and add the milk and fish. As soon as the milk shows signs of coming to the boil, reduce the heat so that it’s simmering. Cover with the lid and cook for 5 minutes. Remove the fish from the milk and set aside on a plate to cool. Pour the milk into a jug for using in the cheese sauce.
Now make the cheese sauce - melt the butter in the same pan used to cook the fish over a low heat. Stir in the flour as soon as the butter has melted. Cook for 30 seconds, stirring constantly.
Slowly pour the milk into the pan, just a little at a time, stirring well until the sauce is smooth and thickened slightly. Bring to a gentle simmer and cook for 3 minutes. Add the grated extra mature cheddar and stir until all melted. Season with salt, to taste, and remove from the heat.
Flake the cooled fish into large chunks and spread evenly in the pie dish (my pie dish is 29cm x 23cm), then pour over the cheese sauce. Set aside for around 5 minutes for the sauce to cool slightly.
Top the fish and cheese sauce with the mashed potatoes and scatter over the grated red leicester grated cheese.
Bake on the preheated tray in the centre of the oven for 25 minutes, turning half way until the potatoes are golden brown all over.
Enjoy with some green veg on the side!