Bring the butter to the boil in a small saucepan, stir in the flour well and remove from the heat. In a larger saucepan, heat the milk and cream. Stir in the processed cheese. Fold in the melted butter. Add the cheddar and simmer briefly until it thickens a little. Peel and quarter the potatoes. Add the stock. Simmer everything together with the potatoes for 25 minutes. At the end, add the rosemary and pepper flavourings. Season to taste with salt and pepper. Serve with fresh chives, fried bacon and fried onions.