Lightly oil a 600ml loaf tin then line with clear film.
Break 100g of the chocolate into a small bowl and either melt it in the microwave or place it over a pan of simmering water. Cool slightly.
In a large bowl, whip the cream and 2tbsp of the icing sugar together with an electric whisk until it forms soft peaks. Use a teaspoon to drizzle the melted chocolate over the surface of the cream. Gently fold into the mixture with the 175g of the raspberries. Spoon into the prepared tin then freeze for at least 3 hours or until frimly frozen.
To serve, remove the terrine from the freezer and leave to stand for 20mins. Whizz the remaining raspberries in a food processor and sweeten with the reamaining icing sugar; transfer into a jug. Invert the terrine onto a serving platter. Melt the remaining chocolate and drizzle over the terrine. Serve in slices with the sauce and more raspberries if liked.