In reply to ChrisGroves:
Write down, preferably in importance the main aspects of their job - the things you will expect them to do. Add a comment at the end something like, "this list is not exhaust and you will be asked to do other jobs/tasks when required.
For the Job spec, describe the kind of skills, knowledge &/or experience you'd expect a competent stores person to have.
Neither of these would be more than an A4 page for this kind of role.