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 pudding basin into the slow cooker, then pour boiling water into the slow cooker pot — about halfway up the sides of the pudding basin.
Cook for 2.5-3 hours, until a knife inserted into the centre comes out clean. Then remove the pot from the slow cooker. Loosen around the edge of the pot with a knife and place a plate on top. Flip over onto the serving plate, then slice and serve.