Put your slow cooker on High.
Grease a 1.2 litre/2 pint pudding basin with butter. I use the plastic lidded bowls that are made for steamed puddings as i find them easier all round. But if you are using a ceramic pudding bowl then seal the top with pleated paper and silver foil as above.
Beat the butter and sugar together until pale and thickened a little. This takes a couple of minutes.
Whisk or beat in half the beaten eggs followed by half the flour for a couple of minutes.
Add the remaining eggs and flour all at once as well as the milk. Whisk or beat again for another minute.
The batter should be a soft dropping consistency. Add a splash more milk if it's not.
Spoon the lemon curd into the bottom of your pudding bowl.
Carefully spoon the sponge mixture into the bowl so that it stays on top of the lemon curd.
Smooth over and place the lid or your coverings over the top.
Place the pudding bowl carefully in the slow cooker and pour just boiled water into the slow cooker bowl so that it comes ¾ of the way up the pudding bowl.
Place the lid on and cook for 3 hours on High. If using a saucepan to steam your pudding then steam for 2 hrs 30 minutes.
To serve take it carefully out of the slow cooker or saucepan. Remove the lid and invert the pudding carefully onto a large serving plate. To do this I put the plate on top of the bowl and very carefully rotate so the plate is on the bottom and then slowly lift the bowl clear.