Have ready your slow cooker according to its instructions.
Grease a 1 pint heatproof pudding basin and place the mincemeat into the bottom.
100 g Mincemeat
Using an electric mixer, beat together the butter and sugar until light and fluffy. Gradually add the eggs one at a time, and vanilla extract if using, adding a little flour between each egg to help prevent curdling.
100 g butter, 100 g soft brown sugar, 2 medium eggs, ½ teaspoon Vanilla extract
Add the remaining flour, mixed spice and mix until well combined.
175 g self raising flour, ½ teaspoon ground mixed spice
Add the Mincemeat and carefully mix until just combined.
100 g Mincemeat
Spoon the mixture on top of the Mincemeat that is already in the pudding basin and level the top.
Cover securely with foil or grease proof paper, making sure there are no holes for any excess steam to drop through onto the sponge batter.
Put the basin in the slow cooker and pour round sufficient boiling water to come halfway up the sides of the basin. TIP: Only do this with a Sear and Stew inner basin, if you add boiling water to a cold ceramic inner pot, it may crack. Some slow cookers allow you to pre heat the inner pot. Check your slow cooker manual for details.
Place the lid on your slow cooker and cook the pudding on high for 3 - 4 hours..
After 2 - 3 hours check to see if the sponge has risen. Carefully lift the foil from the top of the pudding basin and check with a skewer if needed. It should come out clean. If needed return to a further steaming time.
When the cooking time is up, carefully lift the pudding out of the slow cooker and leave it to cool a little before turning it out onto a serving plate.