I was hoping that the Derby City Council website might have information to cover your question but, alas, it does not. Try a phone call to 0345 600 1982. If that turns out to be an automated payment service though (which you can't use), call 01332 640000 instead.
As Dave has said though, it might turn out that some mischievous person has simply attached an empty ticket bag, that they'd found on the pavement, to your windscreen. (Let's be honest. If I found such a bag, and there was a car parked where it shouldn't be, it's probably what I'd do!)