For the caramelised pecans, lightly oil a non-stick baking sheet and scatter over the pecans.
Heat the sugar in a frying pan without stirring until it forms a dark caramel. Pour the caramel over the pecan nuts. Allow to cool slightly, then break up into pieces.
For the gateau, soak the raisins in one tablespoon of the rum.
Place a chefs' ring onto a serving plate. Break the ginger cake into pieces and press into the bottom of the ring. Drizzle over the remaining tablespoon of rum.
Place the sliced banana on top of the rum-soaked cake.
Place the double cream into a large bowl with the icing sugar and whisk until soft peaks form when the whisk is removed. Fold in the custard, a squeeze of lemon juice and the raisins, along with the rum they were soaked in.
Fill the ring to the top with the cream mixture and level off with a knife.
Use a mini-blowtorch to heat the sides of the ring to loosen it, then remove the ring.
To serve, sprinkle over the caramelised pecans.