Does the ticket have the words "Civil Parking Charge Notice" on it? If so, DO NOT follow the advice on the back which is to send your payment and your letter of appeal together. Your appeal will be denied and you'll never see your money again.
These firms are the scum of the earth. I've been battiling with one since last November and I will pay their ticket when hell freezes over. They are operated by private firms and are nothing to do with the police or the council.
If you've done nothing srong, then DON'T PAY IT!
This website has some good tips.
Good luck.
http://forums.pepipoo.com/index.php?autocom=ib wiki&cmd=article&id=56