Preheat the oven to 180°C (fan). Line a traybake tin with baking paper.
Preparing the fruit – Mix the peaches, raspberries, and blueberries with the sugar and cornflour. Set aside while preparing the sponge.
Preparing the sponge – Beat the butter and caster sugar together until light. Add the eggs, one at a time, mixing well after each addition. Add the vanilla and almond flavourings.
Fold in the self-raising flour and ground almonds until you have a soft cake mixture. Add the milk if needed to loosen the mixture.
Assembling the cake – Spread the sponge mixture evenly in the tin. Scatter the fruit over the top, gently pressing some of it into the mixture.
Preparing the crumble – Rub the cold butter into the flour and ground almonds until the mixture resembles breadcrumbs. Mix in the brown sugar, then spread the crumble evenly over the fruit. Finish by scattering the sliced almonds on top.
Bake for 45–55 minutes, until the cake is golden, the crumble is crisp, and a skewer inserted into the centre of the sponge comes out clean. Leave the cake to cool in the tin before cutting it into squares.