If you scroll down on this link you will see here is only 1p between the main supermarkets. Try a different size pack and you might get a better result.
Thanks boxy it is. It even tells you when the special offers finish. I can save over £3 a bag on cat litter by choosing the right supermarket and always check who has the best offer on cat food before I set out. As I might buy 6-8 boxes of cat food while it's on offer it's worth checking.