Preheat the oven to 220C/gas 8. To make the filling, combine the flour, thyme and rosemary together and season with salt and freshly ground black pepper. Toss with the lamb and shake of any excess.
Melt the butter in large saucepan and fry the lamb for 5 minutes or so, until browned. Remove from the pan and set aide. Add the carrot and onion to the pan and cook gently for about 10 minutes, until softened. Add the mushrooms and cook for a further minute.
Return the lamb to the pan and add the port and lamb stock. Bring to boil and cook for 30 minutes, over a moderate heat. Remove from the heat and leave to cool.
Fold in the cheese and mace. Roll half of the pastry out and line a 25cm pie plate; spoon the filling over. Roll the remaining pastry out to form a lid. Brush the pastry with beaten egg and bake for 25-30 minutes, until golden.
Reheat from frozen or allow to defrost and warm through in the oven. Serve with buttered spring vegetables.