Firstly, place the Rice Krispies cereal into a large bowl and set aside.
Then melt the milk chocolate and butter in a medium saucepan, over a very low heat, stirring often. Once melted, add the marshmallows and stir often until melted and combined. Take off the heat and pour into the Rice Krispie bowl, and stir to combine. Ensuring the Rice Krispies are completely covered. Then leave to cool a little (5 minutes or so) until you can handle the mixture.
Using your hands shape the mixture into 18 balls, about 1tbsp mixture per ball, and place onto a baking tray to set. Then place the tray into the fridge for 15 minutes to firm up, while you start on the decorations.
Dust your work surface with icing sugar, and roll out the green fondant using a rolling pin. You want the fondant to be a few millimetres thick. Then using a holly cookie/icing cutter, cut out roughly 36 holly shapes - enough for 2/3 for each pudding ball. Then roll the red fondant into little balls - around 30 or so.
Finally, melt the white chocolate in a bain-marie (or in a microwave in 30 second increments) and place into a piping bag or a small freezer/sandwich bag. Leave to cool for a few minutes, then snip off the end to make a small hole. Pipe on top of each of the balls, coaxing the chocolate down the sides to mimic the icing on a Christmas pudding. Then place holly and berry decorations onto each ball and leave to set.