I don't use syrup, I just melt down a big bar of chocolate (in a bowl over a saucepan of boiling water) then add the rice crispies (or cornflakes). Set in fridge for an hour or so and hey presto! Not a very exact science, just enough to coat the cereal!
Not sure how long they keep (mine never last very long!) but as an additional E number feast, you could make one big crispie cake in a dish, wait for it to set and then pour over an angel delight mix and wait for THAT to set. Dread to think how many calories/E numbers there are in that, but boy does it taste great!!