To make sauce: Combine sugar and water in heavy large saucepan over medium heat until sugar dissolves. Mix in golden syrup and lemon juice. Increase heat and boil without stirring until syrup turns deep amber. Continue to brush down sides of pan with a wet pastry brush and swirl the pan occasionally. Remove from heat; pour in cream (mixture will bubble up) and stir over low heat until caramel is melted and smooth. Increase heat and boil until sauce is reduced by 1/4, stirring often, about 4 minutes. Remove from heat; mix in pecans and bourbon.
To make pudding: Preheat oven to 180 degrees C.
Combine milk, cream and sugar in heavy large saucepan over medium high heat, stir until sugar dissolves and mixture comes to the boil. Remove from heat, add chocolate and stir until smooth.
Beat eggs and vanilla in large bowl to blend. Gradually whisk in chocolate mixture; add bread cubes and let stand until bread absorbs some of the custard, stirring occasionally, about 30 minutes. Transfer mixture to a 23x33cm glass baking dish. Cover with aluminium foil.
Bake in preheated oven until set in centre, about 45 minutes. Serve pudding warm or at room temperature with warm sauce.