Flapjack apple crumble

Name: Flapjack apple crumble
Ingredients: 250g/9oz porridge oats; 2 tsp lemon zest; 125g/4½oz unsalted butter; 100g/3½oz muscovado sugar; 100g/3½oz golden syrup; 1kg/2lb 4oz Granny Smith or cooking apples, peeled, cored and cut into chunks; 40g/1½ oz unsalted butter; 1 tsp cinnamon; 1–2 tbsp muscovado sugar; 1 lemon, juice only; 1 tbsp cornflour; 150g/5½oz raisins; 600ml/1 pint whole milk; 1 tbsp vanilla extract; 4 free-range egg yolks (roughly 80g/3oz); 20g/¾oz caster sugar; 2 tsp cornflour