Trim and wash the leek. Cut into 3 to 4 Pieces; cook in boiling water for 10-20 mins until just soft; drain and allow to cool; meanwhile cook cheese sauce as directed on packet. When the leeks are cool enough to handle wrap a few slices of the wafer thin ham around each and place evenly spaced between in a small ovenproof dish. Pour on the cheese sauce. Then make up the instant mash as directed on the packet adding a knob of butter or margarine. Then spread enough mash evenly on top of the leeks and sauce dish to cover. Scrunch up the crisps in the bag and pour on top for that extra crispy coating (optional). Bake in the oven gas mark 5-6 for 20-30 mins or until the top is brown and the sauce is bubbling.