1. Pre-heat oven to 190. Line and grease a round cake tin.
2. Place the chopped rhubarb in a bowl and add the caster sugar and elderflower, mix and leave for about 30 mins. Place in a dish and pop in the oven, covered for 15 mins, take out and leave to cool.
3. Next make the banana cake. Mash the bananas in the food processor, then add the sugar, butter, eggs, flour, and baking powder. Whiz till well blended.
Add the milk and whiz again.
4. Poor mixture into tin, then spoon the rhubarb on top.
5. Bake for about 50 mins, cook on rack.