Preheat the oven to 200°C.
Peel the potatoes and cut into 2cm chunks, then boil for around 15minutes, or until tender.
Meanwhile, make the sauce. Melt the butter in a saucepan over a low heat and stir in the flour. Add the bay leaves. Then gradually add the stock and the milk, stirring after every addition until the sauce is smooth and silky. Bring to the boil, then reduce to a simmer and cook for 10 minutes, or until thickened.
