Just got mine out of the oven. Super easy

1.2kg potatoes peeled and sliced
2 1/2 cups milk
60g butter
1/4 cup flour
2 cups cheese
2 rashers bacon- rind removed and diced.

Melt butter, remove from heat, whisk in flour. Add milk bit by bit stirring constantly. Return to heat, stir continuously till sauce thickens. Add 1 1/2 cups cheese and stir till combined. Add salt and pepper.
Layer 1/3 potato slices on bottom of tray slightly overlapping, top with 1/3 bacon then 1/3 sauce.
Repeat till ingredients finished. Sprinkle with remainder of cheese and bake for 45 mins on 200 degrees celcius.