Position an oven shelf in the centre of the oven. Preheat the oven to 180°C/fan oven 160°C/Gas Mark 4. Grease an 18cm (7 in) round loose-based cake tin with a little Trex, then line with greaseproof paper.
Mix together the flour, porridge oats and salt, then use your fingertips to rub in the Trex until the mixture resembles fine crumbs. Stir in the muscovado sugar and spice.
Tip half the mixture into the prepared cake tin and press it down lightly. Arrange half the apple slices and blackberries evenly on top. Sprinkle the remaining crumble mixture over them, press down lightly, then add the remaining fruit, spreading it over the surface. Sprinkle with the demerara sugar.
Bake for 35-40 minutes until firm and golden brown. Stand the tin on a wire rack and cool for 15-20 minutes, then remove from the tin. Carefully remove the lining paper. Serve warm or cold with ice cream, custard, crème fraiche or yogurt.