Preheat the oven to 180°C (160°C for fan assisted ovens), gas mark 4. Grease and line 2 x 900g loaf tins with nonstick baking paper.
Mix the eggs, oil, water and cake mix in a large bowl and whisk (by hand or using an electric mixer) for 2-3 minutes, until smooth and creamy. Mix in the banana, 200g blueberries and the oats.
Divide the mixture evenly between the tins and smooth the tops. Scatter over the rest of the blueberries, gently pushing into the sponge.
Make the crumble topping. In a medium bowl, rub the flour and butter together with your fingertips (or whizz in a food processor), until the mixture is the texture of breadcrumbs. Stir in the oats and sugar.
Spoon the crumble mixture over the top of the loaves and bake in the oven for 1 hour 20-25 minutes, or until a knife inserted into the loaves, comes out clean.
Remove from the oven and leave to cool for 10 minutes before turning out, crumble side up, onto a wire rack to cool completely. Slice and serve.