Peel and thinly slice the potatoes. Slice the leeks and bacon, removing any excess fat.
Melt butter in a saucepan, add flour and cook for a minute. Gradually add milk, stirring until you have a smooth, thickened sauce. Add half of the cheddar. Continue to cook for a few minutes.
Layer the potato, leeks, and bacon in an ovenproof casserole dish with some of the sauce. Spoon the remaining sauce over the top, level and sprinkle with the rest of the grated cheese. Cook for 35-40 minutes. or until golden and bubbling. Check the potato is cooked through by inserting a spoon or knife into it.