Preheat your oven to 180°c (160°c for fan-assisted ovens, Gas Mark 4 or 350°F). Lightly grease a 20cm (8-inch) loose-bottom tart tin. Set aside. In a large bowl, start by creaming together the caster sugar and butter/margarine, until light and fluffy. Add the egg and mix through until mostly combined (don't worry if it looks slightly curdled, simply add a spoonful of your flour to overcome this). Add the flour and mix to create a dough - you may need to use your hands.
Transfer your pastry dough to a lightly floured work surface and gently knead until you dough is smooth.If your dough feels a little too soft, you can wrap it in clingfilm and allow it to firm up in the fridge for 20 minutes - the need to do this will generally depend on the temperature/humidity of your kitchen. Roll your pastry dough into a large circle, until it is a few centimetres bigger than the diameter of your tart tin (so around 26-28cm). Carefully transfer the pastry dough to your prepared tart tin, being careful not to catch too much air underneath, and gently pressing into the sides. Leave a little overhang on the pastry but trim off any large bits of excess overhang. Prick the bottom of your pastry with a fork, before lining the pastry with some greaseproof/parchment paper and filling with baking beans. Blind bake the pastry for 15 minutes, before carefully removing the paper and baking beans (watch out, they will be hot!) and return the tart pastry to the oven to continue baking for a further 5-10 minutes, until it is a light golden colour. At this stage, trim the remaining excess pastry overhang to leave a neat edge on your tart.
Once the pastry is baked, spread your jam within the pastry when it is still warm, then set aside while your prepare your cornflake filling.
In a large pan, melt the golden syrup, butter/margarine and brown sugar. Once completely melted, remove from the heat and stir in the cornflakes. Stir really well (but gently, although some breakage is to be expected) until all the cornflakes are coated.
Spoon the cornflake mixture on top of your jam-filled pastry, ensuring an even covering. Return to the oven and bake for a final 5-10 minutes, until the cornflakes are golden and toasted.
Remove from the oven and allow to cool until just warm, and then remove from the tin. Serve in slices with custard (optional).