Mine are currently using up some leftovers of Harry Hamster minus the extruded wheat biscuits, Science Selective and Pets at Home Nuggets. There's also a little bit of a premade seed mix I bought too and it seems to go down better than any of the pre-prepared mixes. It might not be 100% balanced but James is looking in such good condition at the moment.
They get a couple of seeds as a treat plus dried mealworms which I buy loose from a local pet shop as well as the odd dog biscuit. My box of hamster food stuffs is pretty full at the moment, they have more than I do!