Heat oven to 190C (170C fan). Rub the butter into the flour until it looks like breadcrumbs, stir in the sugar and add enough cold water to bring it together into a dough. Roll out and line a 20 cm tart tin, prick the base and chill for 10 minutes.
Line the pastry with baking paper and baking beans, and bake blind for 12 minutes. Remove the paper and beans and bake 5 minutes more until the base is dry and pale gold. Reduce the oven to 180C (160C fan).
Warm the jam gently and spread it over the base in an even layer.
Melt the 50 g of butter with the golden syrup in a large pan over a low heat, then take off the heat and fold in the cornflakes gently, keeping some crunch, until every flake is coated.
Pile the cornflake mixture over the jam and bake for 10 to 12 minutes until the topping is shiny and just beginning to darken at the tips. Cool completely in the tin before slicing into 8, the jam layer needs to stop bubbling first.