Preheat the oven to 200C/180C fan/Gas 6. Put a baking tray in the oven to heat.
Make the mashed potatoes - put the potatoes in a medium saucepan of cold water, bring to the boil and add some cooking salt. Cook on a continuous simmer for 15 to 18 minutes until the potatoes are tender.
Drain well in a colander and let them sit for a minute or 2 to steam dry. Meanwhile, add the 70ml of milk and 35g of butter to the saucepan and put on a low heat to warm the milk and melt the butter. Turn off the heat and add the cooked potatoes. Mash well until the potatoes are smooth and the milk and butter combined. Season with black pepper, to taste – stir again well to combine the seasoning. Set the pan of mash aside until ready to top the pie at step 8.
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 600ml of milk and fish (if your fish fillets are quite large or long, cut them in half to fit into the pan). 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 it 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 75g of 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.
Increase the heat slightly, then slowly pour the reserved jug of milk into the pan, just a little at a time, stirring well until the sauce is smooth and thickened slightly. Bring it to a gentle simmer and cook for 3 minutes. Add 200g of the grated cheddar and stir until all melted. Season with salt, to taste, if needed and remove from the heat (I tend to find I don't need to add salt to the cheese sauce due to the smoked fish being cooked in the milk, and with using extra mature cheddar).
Remove any skin from the fish and discard it. Flake the cooled fish into large chunks and spread evenly in a pie or ovenproof dish (I usually use my medium rectangular dish that's 27cm x 20cm), then pour over the cheese sauce. Set aside for around 10 minutes for the sauce to cool slightly.
Top the fish and cheese sauce with the mashed potatoes and scatter over the 75g of grated cheddar.
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 your choice of vegetable side dishes!