Beat the butter or margarine, sugar, flour, eggs and cinnamon together in a bowl or combine in a food processor until smooth. Stir in 50 g (2 oz) of the hazelnuts. Grease a roasting tin with a base measurement of 23 x 18 cm (9 x 7 inches) and line the base with nonstick baking paper. Spoon the mixture into the tin and smooth into an even layer.
Arrange the pears randomly over the pudding mix, then sprinkle with the remaining hazelnuts. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25 minutes until golden brown and the sponge is well risen and springs back when pressed in the centre.
Cook 150 g (5 oz) of the blackberries in a saucepan with the sugar and water for 5 minutes until soft, then purée until smooth. Cut the sponge into portions, dust with sifted icing sugar and serve drizzled with the warm sauce, with the remaining blackberries.