Mary Berry Apple Crumble

Name: Mary Berry Apple Crumble
Prep time: 10 min
Cook time: 40 min
Total time: 50 min
Cuisine: British
Ingredients: 5 oz (1.5kg) apples, like Bramley or Granny Smith to get the right acidity and texture; ½ lemon, juice only; ½ cup (100g) light muscovado sugar; 6 tablespoons water; 6½ tablespoons (50g) plain flour; 1/3 cup (50g) porridge oats; 3/4 cup (150g) light muscovado sugar; 5½ oz (150g) walnuts, roughly chopped; 1/3 cup (50g) sunflower seeds; 3½ oz (100g) unsalted butter, softened