1. Preheat the oven to 200 (180 fan, gas mark 6) 2.Chop the chicken into chunks and pop into a large frying pan with the olive oil on medium heat. 3. Wash and slice the leeks into slices around 1.5 cm thick. Add to the frying pan with the chicken and cook until softened and the chicken is cooked through. Remove from heat. 4. Slice the potatoes (skin on) 1 cm thick and pop into a saucepan of water. Bring to the boil and leave for 2 mins before draining. You want them to be softened but not falling apart. 5. Melt the butter, with the crushed garlic, in a small saucepan on medium heat. Mix in the flour until it makes a paste and allow to cook through for a minute or two, stirring occasionally. 6. Slowly add the milk, stirring as you go and allowing to thicken. You want it to have some thickness to it, but it must be runny. You can add the optional grated cheese here if you wish. Stir through until melted. 7. Transfer the chicken and leeks to a large casserole dish. Add the frozen peas and stir them through. 8. Pour the garlic sauce over the ingredients in the casserole dish, season to taste and stir through a little. It should just cover them. 9. Layer the potato slices over the sauce, each one slightly overlapping. 10. Spray the potatoes with spray oil. 11. Pop into the oven for 25-30 minutes or until the potatoes are browning on top. 12. Serve.