Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the flour, suet, sugar, lemon rind and a pinch of salt in a bowl and mix well. Gradually stir in the milk or milk and water to make a soft but not sticky dough. Knead lightly, then roll out on a piece of floured nonstick baking paper to a rectangle about 23 x 30 cm (9 x 12 inches). Turn the paper so that the shorter edges are facing you.
Spread the jam over the pastry, leaving 2 cm (¾ inch) around the edges. Roll up, starting at a shorter edge, using the paper to help. Wrap in the paper, then in a sheet of foil. Twist the ends together tightly, leaving space for the pudding to rise.
Transfer the pudding to the slow cooker pot and raise off the base by standing it on 2 ramekin dishes. Pour boiling water into the pot to come a little up the sides of the pudding, being careful that the water cannot seep through any joins. Cover with the lid and cook on high for 3½–4 hours or until the pudding is light and fluffy. Lift out of the pot, then unwrap and cut into thick slices. Serve with hot custard.