Cut the jelly into cubes and place in a large jug. Heat the mulled wine to just below boiling and pour over the jelly, stir to dissolve and top up with water to give 500ml.
Place the cake slices in the base of a 1.2 litre trifle dish and pour over the jelly, chill until set.
Meanwhile, place the custard powder, 50g sugar and nutmeg in a saucepan and blend in the milk, bring to the boil, stirring continuously until thickened. Chop half the walnuts and add to the custard. Transfer to a bowl, cover the surface with clingfilm and allow to cool.
Meanwhile, place 50g sugar in a small saucepan with 50ml water, bring to the boil then add remaining walnuts, cook for 2 minutes. Place the remaining sugar on a plate. Using 2 forks remove the walnuts from the syrup and toss in the sugar, allow to cool.
When the jelly is set, spread over the custard. Whisk the cream to soft peaks and spoon over the custard.
Sprinkle over the sugared walnuts and sprinkles to serve.
Notes
Cooks tip Try different flavours of jelly and serve with a sparkler for really festive fun.