1. A distributed storage network comprising a plurality of interconnected computers, said computers including a plurality of host computers, each having a store for data items, and at least one client computer, wherein said client computer stores client code comprising: a) storage initiation code executable to initiate storage of a data item in one or more of said plurality of host computers' stores; b) storage condition generation code executable to generate one or more interpretable storage conditions which must be met by a candidate host computer store in order for said candidate host computer store to store said data item, said interpretable storage conditions indicating a required value of one or more time-varying storage characteristics; c) storage condition sending code executable to send said one or more interpretable storage conditions to one or more of said plurality of host computers; wherein each of said host computers stores host code including: d) storage condition reception code executable to receive said one or more interpretable storage conditions from another of said plurality of host computers; e) host computer store characteristic provision code executable to provide host storage characteristic data indicating the value of one or more time-varying characteristics of said host computer's stores; f) storage condition interpreter code executable to interpret said one or more interpretable storage conditions in the light of said host storage characteristic data provided by said host computer and thereby to establish whether said host computer's store meets said storage conditions; g) data item storage code executable to store said data item in said host computer's store on the execution of said condition interpreter code finding that said host computer meets said storage conditions; and h) storage condition forwarding code executable, on the execution of said condition interpreter code finding that said host computer's store does not meet said storage conditions, to forward said storage conditions to another of said plurality of host computers.