Preheat oven to 180°C (350°F/Gas Mark 4) and prepare a 20cm round cake tin with parchment paper.

Cook chopped rhubarb, granulated sugar, and water in a saucepan over medium heat for 5-7 minutes until syrupy.

Cream softened butter and caster sugar until light and fluffy. Add eggs one at a time, mixing well.

Sift self-raising flour over creamed mixture and fold in gently. Add milk to adjust consistency if needed.

Layer half the batter in the tin, swirl in cooled rhubarb mixture, add remaining batter, and more rhubarb swirls.

Bake for 40-45 minutes until golden brown and a skewer comes out clean. Cool for 10 minutes before transferring to a wire rack.