Preheat oven to 190 deg C. Wash rhubarb and slice finely (a food processor makes short work of this). Slice bananas and mix with rhubarb, sugar and custard powder. Spread into a 23 cm pie plate.
Mix together in a food processor (or sift), flour , cornflour, baking powder and caster sugar, then process (or rub) with butter to a fine crumb. Mix into a dough with egg and water, roll out and cover rhubarb, sprinkle with raw sugar. Bake at 190 deg C for 20 minutes then reduce to 170 deg C for a further 25 minutes. Serve warm or cold with whipped cream or custard.