Preheat the oven to Gas Mark 5, 190°C, fan170°C. Line a shallow 20cm (8in) square cake tin with nonstick baking paper.
Place all the ingredients, except the apple and blackberries, into a large mixing bowl and beat with a wooden spoon until smooth.
Stir in the apple and blackberries and spread into the tin. Bake for 30-35 minutes until golden and the centre is firm to touch.
Leave in tin for 10 minutes before turning out and allowing to cool completely. Cut into squares. Keeps for three days.
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