Make the jelly by whisking together the water, food dye and Natvia until it starts to bubble. Add the gelatine and vanilla combine. Pour into the trifle bowl. Cover and refrigerate for at least 1 hour.
Whisk together the Natvia, flour, milk and butter in a mixing bowl. Pour into a greased and lined 15x20cm cake tin. Bake for 15-20 minutes, until a skewer inserted comes out clean. Allow to cool completely on a wire rack. Break the cake up into small pieces.
Make the custard by heating ¼ cup of the milk with the custard powder over medium high heat. Add the Natvia and remaining milk and stir. Stir continuously and heat for 2-3 minutes until thickens.