This has nothing to do with parking procedure, it is, as the council's response states, a contractual matter between Enterprise and you. From your account, the issue is whether the last hirer may be held liable for a contravention arising directly out of them leaving the car in a prohibited location/position even when that person's hire agreement terminated* before a PCN was issued.
*- this is a legal issue. It is common practice with some forms of agreement that parties may be bound by enduring conditions even after the term of a contract has expired.