When I make Nigella's London Cheesecake the biscuit base often gets very hard almost that cannot be be sliced easily. This happens with some other cheesecakes too. Can you suggest a solution?
If the digestive biscuit (graham cracker) crumb base of a cheesecake is very difficult to cut then it is most likely that the base has been compressed too much when it is put in the bottom of the pan. When the base is baked and/or chilled then it sets very hard and can become very difficult to cut through.
When you have mixed the crumb base, tip it into the pan and spread it out evenly (a palette knife/metal icing spatula is good for this). Press the crumbs onto the base lightly using your fingers, making sure that you don't press too hard on the crumbs. For a smooth finish you could instead use the base of a glass or a metal mesuring cup, but again don't press too hard. Remember that the base will set more as it is cooked/chilled so the crumbs don't have to be packed completely solid.
If you find that the crumb base is still too hard, then you could slightly reduce the amount of butter used, though we would advise caution on this as the base could become too crumbly instead.