Put the rhubarb into a saucepan with the caster sugar, cover and simmer on a low heat for 15 minutes. When soft, pour the rhubarb into a medium baking dish.
Heat oven to 200°C/180°C fan/gas mark 6. To make the topping, rub the flour and butter together with your fingers until you have a soft, crumbly mixture. Add the brown sugar, mixing together with your hands.
Scatter the topping over the rhubarb and bake for 30 minutes or until golden brown on top. Serve piping hot with custard.