Preheat and Prepare: First things first, preheat your oven to 180°C (160°C fan). Grease and line a 20cm (8-inch) square baking tin with baking parchment, leaving some overhang to help you lift the bars out later.

Make the Crumble Mixture: In a large mixing bowl, combine the plain flour, rolled oats, light brown sugar, bicarbonate of soda, ground cinnamon, and salt. Give it a good whisk to mix everything evenly.

Add the Butter: Add the cold, cubed butter to the dry ingredients. Using your fingertips, rub the butter into the flour mixture until it resembles coarse, damp breadcrumbs. You can also do this in a food processor using the pulse setting, but be careful not to over-mix.

Form and Pre-bake the Base: Take about two-thirds of the crumble mixture and press it firmly and evenly into the base of your prepared tin. I find that using the back of a metal spoon really helps to compact it down. Bake for 10-12 minutes, just until it's set and very lightly golden at the edges.

Prepare the Cranberry Filling: While the base is baking, make the filling. In a medium saucepan, combine the cranberries, caster sugar, orange zest, and 40ml of the water. Cook over a medium heat, stirring occasionally, for 8-10 minutes. The cranberries will burst and the mixture will thicken into a vibrant, jammy sauce.

Thicken the Filling: In a small bowl, whisk the remaining 20ml of water with the cornflour to create a smooth slurry. Pour this into the cranberry mixture and stir constantly for another minute until the filling is glossy and noticeably thicker. Remove from the heat.

Assemble the Bars: Once the base is out of the oven, carefully pour the warm cranberry filling over it and spread it into an even layer with a spatula.

Add the Topping and Bake: Sprinkle the remaining one-third of the crumble mixture evenly over the cranberry layer. Don't press it down. Return the tin to the oven and bake for a further 25-30 minutes, or until the topping is a beautiful golden brown and the filling is bubbling gently at the edges.

Cool Completely: This is the most important step! You must let the bars cool in the tin completely, first at room temperature and then in the fridge for at least an hour. This allows the filling and butter to set, ensuring you get clean, neat slices.

Slice and Serve: Once fully chilled, use the parchment paper overhang to lift the entire block out of the tin. Place it on a cutting board and use a sharp knife to cut it into 16 squares.