Place condensed milk in a mixing bowl and add lemon juice 5ml at a time while mixing thoroughly. Taste after every 5ml and add to suit your taste. Lemon juice will curdle the condensed milk and it will finally turn lighter in colour and change the texture. Be sure to end with a fairly tart (tangy) mixture to offset the sweetness of the custard. Must be pleasant to eat on it's own though -- Put aside but do not refrigerate.
Make approximately 500 ml of Custard.
Place one layer of Tennis biscuits on the bottom of a 20 x 30 glass or ceramic fridge/oven dish. Spread a layer of Custard (approximately 1cm) over the biscuits without disturbing them but covering them. Place another layer of biscuits over the custard. Now spread a layer of the lemony condensed milk over the biscuits (approximately 5-6 mm). Repeat until you run out of ingredients or you reach the top of the dish.
Leave one biscuit for the end to crush and sprinkle over the top. You can decorate the top with other sprinkles but biscuit crumbles are the original. Refrigerate for 4 to 6 hours but cover with tin foil to keep fridge flavours out. Cut in square portions and serve.