Preheat the oven to 180ºC.
Grease a 1.2L pudding basin with reduced-fat spread. Cut a circle of non-stick baking paper to fit into the bottom of the dish. Spoon the jam on top, then roughly spread out.
Add the flour, spread, sweetener, caster sugar, eggs, and baking powder to a mixing bowl. Whisk with an electric whisk for 2 minutes — until light and creamy.
Scrape the cake mixture into the bowl, on top of the jam layer.
Cut a 28cm circle of non-stick baking paper and tear off a section of foil, slightly bigger than the circle. Layer the paper, then foil, and wrap the top of the dish tightly with the paper on the inside.
Place the basin into the oven and cook for 45 minutes, until risen and a knife inserted into the centre comes out clean. Then remove the basin from the oven. Loosen round the edge of the basin with a knife and place a plate on top, then flip over onto the serving plate. Slice and serve.