Beat butter and sugar together, add flour, milk, and lemon rind. Combine and pour into 3 litre slow cooker
Sauce:
Mix water, lemon juice and sugar together and gently pour over cake mixture.
Cook on high with tea towel under lid for 1 1/2 - 2 hours. Once cooked, switch off SC and Let stand for 5 - 10 minutes to allow sauce to thicken before serving.