Put anything edible up on shelves (so everything) put poisons in the shed where rat can access it, "Neosorexa" if you can get it, its readily available in farm shops. dont skimp on it!
It is safe to put it under the shed. The poison consumed by the rat will not affect any cats that were to eat the rat, so dont worry. You dont get secondary poisoning with most modern rodenticides. The easiest way to get it under a shed is to put poison in plastic bags and push it under with a stick, rats will soon chew into the bag!