Changing a cat's food all of a sudden can make them sick and hork up on your rug. Letting them eat the same type of food every day for twenty years is just fine. That's what they like. If you find something they'll eat, feed that to them and everyone's happy.
By "biscuits", do you mean like regular dry cat food or are you hand feeding your cat some fun treats by hand to make it do tricks? I'm sure biscuits are the same as cookies (UK vs. USA), but I've never heard of cat food (those little hard bits someties shaped like little plus signs) being called biscuits.
