Cook the bacon under the grill; chop and set aside. Unless you are freezing it - preheat the oven to 180 degrees C / Gas mark 5.
In a saucepan over medium heat, melt the butter and stir in the flour. Whisk for a few minutes until the flour has cooked. Add the stock and bring to the boil. Stir in the leeks and simmer for a few minutes. Add the fresh thyme leaves, peas, chopped bacon and turkey.
Spoon into a casserole dish and top with leftover mashed potato. You can freeze it at this stage or bake in the oven for 20-30 minutes until heated through and potatoes are crispy and golden on top.
If the pie starts to bubble over but the potatoes haven't browned - place the pie under the grill for a few minutes to finish it off.