Photograph: Tara Fisher
Heat the oven to 190°C, fan 170°C, gas 5. Peel, quarter and core all the apples. Thinly slice the eating apples and thickly slice the bramleys. Transfer to a 25 x 17cm ovenproof baking dish. Add the lemon juice; toss together. Drizzle over the golden syrup, then cover the dish with foil and bake for 30 minutes; baste the apples halfway through. Remove from the oven and pour off the syrup into a small bowl.
Scatter the flapjack and fudge over the fruit, drizzle the golden syrup back over and return to the oven, uncovered, for 10 minutes.
Remove the crumble from the oven and leave to stand for 15-20 minutes. Stir the whisky into the custard and gently heat it in a pan. Pour into a jug and serve with the crumble.
Get ahead: prepare up to the end of step 1 a few hours ahead. Reheat gently, then continue with the recipe.