Preheat the oven to 180C/350F/Gas 4. Grease a loaf tin with a little butter.
In a bowl, rub together the oats, flour, sugar and butter using your fingertips, until the mixture is crumbly.
Scoop half of the mixture into the prepared loaf tin and flatten into an even layer using a palette knife. Spread the mashed blueberries on top.
Spoon the remaining crumbly mixture on top of the blueberries, then press down firmly to flatten into an even layer.
Transfer the flapjack to the oven and bake for 18-20 minutes, or until golden-brown. Set aside to cool completely, then cut into slices and remove from the tin.