Prepare the biscuit base: Place the digestive biscuits in a sealed food bag and crush them into fine crumbs using a rolling pin, or pulse them in a food processor until you have a sandy texture with no large pieces. Tip the crumbs into a medium bowl, pour in the melted butter and the 30g of caster sugar, and stir until every crumb is coated and the mixture looks like wet sand. Press this mixture firmly and evenly into the bottom of a 9x13 inch baking dish lined with aluminium foil, using the flat bottom of a glass to compact it well. Place the dish in the fridge to chill while you prepare the filling.

Make the blueberry compote: In a small saucepan, combine the blueberries and the 50g of granulated sugar. Cook over a medium heat for 5-7 minutes, stirring occasionally, until the blueberries have burst and released their juices. You will see the mixture become thick and syrupy, and the colour will turn a deep, rich purple. In a small bowl, whisk the cornflour with the cold water until smooth, then pour this slurry into the blueberry mixture. Stir constantly for another minute until the compote thickens noticeably and coats the back of a spoon. Remove from the heat and allow it to cool completely to room temperature.

Whip the cream: In a large, clean bowl, pour the double cream and vanilla extract. Using an electric hand whisk or a stand mixer fitted with the whisk attachment, whip the cream until it holds soft peaks. When you lift the whisk, the cream should form a gentle peak that flops over slightly. Be careful not to overwhip it, or it will become grainy and start to separate.

Make the lemon filling: In a separate large bowl, beat the softened cream cheese with an electric mixer until it is completely smooth and free of lumps, which should take about 1 minute. Add the sifted icing sugar, lemon zest, and lemon juice, and beat again until the mixture is silky and well combined, with a pale yellow colour and a fresh, zesty aroma.

Fold everything together: Take a third of the whipped cream and stir it into the cream cheese mixture to lighten it. Then, gently fold in the remaining whipped cream using a rubber spatula. Use a figure-of-eight motion, cutting through the centre and bringing the mixture from the bottom up. Fold just until no white streaks remain; the filling should be thick, airy, and mousse-like in texture.

Assemble the bars: Remove the chilled base from the fridge. Spoon the lemon cream cheese filling over the biscuit base and spread it into an even layer using an offset spatula or the back of a spoon. Drop small spoonfuls of the cooled blueberry compote over the top of the filling. Using a knife or a skewer, gently swirl the blueberry compote into the lemon filling to create a marbled effect. Do not over-swirl, or the colours will muddy together.

Chill until set: Cover the dish tightly with cling film and refrigerate for at least 4 hours, but preferably overnight. The bars are ready when the filling feels firm to the touch and no longer wobbles when you gently shake the dish. The longer they set, the cleaner your slices will be.

Slice and serve: Once fully set, lift the bars out of the dish using the foil overhang. Place them on a chopping board and use a sharp, clean knife to cut into 12 even squares. Wipe the knife clean between cuts for the neatest edges.