Preheat the oven to 190°C/375°F/Gas mark 5. Lightly grease a 1.7-litre/3pt pie dish and place on a baking tray.
Cook the potatoes in a pan of salted boiling water for 5 minutes or until partially softened, then drain thoroughly.
Put the cod in a shallow pan. Pour over 50ml/2fl oz of the milk and season lightly with salt and pepper. Cover and poach for 5 minutes until tender. Drain, reserving the liquid. Flake the cod, discarding the skin and bones.
Melt 25g/1oz of the butter in a pan and sauté the leeks for 3 minutes, adding nutmeg to taste.
Melt the remaining butter in another small pan. Add the flour and cook, stirring, for 1 minute. Remove from the heat, and gradually blend in the remaining milk and the reserved fish liquid, stirring until smooth.
Return the pan to the heat. Cook, stirring, until the sauce has thickened. Crumble in the Stilton and add the cream. Season with salt and pepper.
Put the cod, leeks and prawns in the prepared pie dish. Spoon over half the sauce, add the potatoes and pour over the remaining sauce. Cook the pie in the oven for 45 minutes. Serve immediately