I have to feed my two cats seperately. With my adult cat I could happily leave biscuits down all the time and he only eats when he is hungry and he eats very slowly. My kitten, well, she would eat everything in the house in one sitting if she got chance. My adult cat also wont fight for his food, he just lets the kitten eat his share too! So, I shut the kitten in the dining room for fifteen/twenty minutes with her bowl, and leave my adult cat elsewhere with his bowl. They get biscuits morning and late afternoon, and some wet food just before I go to bed.
Its hassle yes, but, it means they both are healthy weights.
As to the litter - it seems your cat knows the difference!! Like someone else said either stick with tescos and be prepared to clean up evey so often, or buy the catsan.
nic