Make up jelly according to packet instructions and leave to set.
To make the custard, mix the custard powder, sugar and 50ml milk. Heat the remaining 450ml milk just to the boil, remove from heat and add the custard mix, stirring well. Return to low heat, stirring continuously until it thickens. Sprinkle with a little sugar to prevent a skin forming, cover with plastic wrap and let cool.
Cut the sponge cake into slices, spread thickly with strawberry jam and arrange slices on the bottom and around the edges of a large glass bowl. Sprinkle with sherry to taste.
Cut up the jelly and mix in chopped pecans. Add in a layer on top of the sponge cake. Spoon the custard on top of this.
Decorate the trifle with whipped cream, glace cherries and the remaining pecan halves. Chill for 1-2 hours before serving.