Welcome likaleica. There's very few places now to escape the money grabbing tentacles of the parking weasel, as you are painfully aware. The following blogsite will give you an encyclopaedic knowledge on how to deflect sPeCulative iNvoices.
http://parking-prankster.blogspot.co.ukMore specifically, these excellent guides,
http://www.parking-prankster.com/court-claim.htmlAlthough the guides are geared toward defence of court action, the legal principals are the same for a POPLA appeal. You will find that Pepipoo and MSE forums are far more experienced than us at assisting defences but we are still happy to help in any way we can. Whether you decide to self learn or to take some assistance, the appeals process is very long and drawn out. Two to three months is not unusual.
If you are the type of person who doesn't want to get too involved and wants it all over in a flash then try this,
http://www.parkingticketappeals.org.ukYou basically pay them £16, send them full details, and then it's no longer your problem.