Preheat the oven to 180°C/350°F/Gas mark 4.
Roll the fillets into coils with the skin on the inside. Secure with toothpicksm and put in a well-greased ovenproof dish.
Combine the stock, wine, shallot, bay leaf and peppercorns in a jug, and pour over the fish. Cover with greased foil and bake for 15 minutes or until the fish flakes when tested with a fork. Carefully lift the fish out of the liquid with a slotted spoon, put in another dish, cover and keep warm.
Pour the cooking liquid into a saucepan, and boil for about 2 minutes until reduced by half, then strain.
In a clean pan, melt the butter, add the flour and stir for 1 minute, or until pale and foaming. Remove from the heat and gradually stir in the milk, cream and strained cooking liquid. Return to the heat, and stir until the mixture boils and thickens.
Add the grapes, then stir until heated through. Serve immediately, with the sauce poured over the fish.