Preheat the slow cooker if necessary; see the manufacturer's instructions. Lightly butter a 1.2 litre (2 pint) soufflé dish and base-line with a circle of nonstick baking paper, checking first that the dish will fit in the slow cooker pot. Arrange the blackberries and plums in the base, then dot with the jam.
Beat the butter and sugar in a bowl with a wooden spoon or hand-held electric whisk until soft and creamy. Gradually mix in alternate spoonfuls of flour and beaten egg, and continue adding and beating until the mixture is smooth. Stir in the almonds and almond extract. Spoon the mixture over the fruit, spread it level and cover the top with foil.
Lower the dish into the slow cooker pot and pour boiling water into the pot to come halfway up the sides of the dish. Cover with the lid and cook on high for 4–5 hours or until the sponge is well risen and springs back when pressed with a fingertip.
Lift the dish out of the slow cooker pot using a tea towel and remove the foil. Loosen the edges of the pudding with a knife and turn out on to a plate with a rim. Decorate with toasted flaked almonds, if liked, and serve hot with custard.