Car Parks (and Parking in them)
Parking a camper in a car park used to be no problem at all, and you'd never give it a second thought, but in the last few years its all got a bit silly. Many car parks have height barriers that simply prevent you from parking in them, and those that don't have signs that are deliberately confusing about what, when and where, so you need to be careful when parking anywhere.
ALWAYS
Read the sign, then read it again, make sure you understand it and the 'rules', they will rely on this sign as their 'contract' so its important. Pay the appropriate charge, if you're spanning two bays, then pay twice the charge, 3 bays, 3x the charge etc. Take a picture on your phone of the Sign, the Machine, Your ticket, and your vehicle, and keep these for at least 3 months.
Bays
Some car parks are beginning to have dedicated bays for campers and motorhomes, and in which case they will probably stipulate that you MUST use these bays, the problem with this is that you can find, and will find an empty car park with the few specific bays in use, and no option to park. This is annoying. If you call the number on the sign, and in the unlikely event you get through to someone, they may tell you that is 'ok' to park elsewhere, but long gone are the days when you can take people at their word, so unless you recorded the call, don't rely on it.
Companies
In recent years car parks have started to use third parties to manage the payments and over-stay invoices, and there are quite a few, but one to be aware of is ParkingEye. It is highly recommended that you take extreme care when parking in a car park that mentions ParkingEye on any sign, they are well known for sending invoices when they aren't applicable, ignoring letters, are almost impossible to deal with and whilst you will almost certainly win at court, it will take up way too much of your time to get there, and you'll never get that back.
Challenging a fine (or invoice)
Even if you follow the deliberately confusing signage, pay the parking, and leave on time, you're still likely to receive a penalty at some point on your travels. Many of these are false but some may be correct if you misunderstood, under paid, or overstayed. Deciding if its a valid penalty or not is vital before taking any decision on how to handle it. If you took pictures as suggested above then you've got all the evidence required right there, but if not, then you need to decide if challenging it is worth your time, and it will take you time.
Council run car parks
Local councils will send you a PCN or Penalty Charge Notice and it should contain instructions on how to appeal it, if not you can find the information on the Gov.uk Website.
Private Companies
This is a murky area since private companies are not regulated in any way, even if they pretend to be, but in essence where as a county council has a 'right' to enforce their penalty charge notices, private companies are simply sending you an invoice for 'breach of contract', that contract being formed when you parked. Now this to most people is nonsense, being able to form a contract by simply being somewhere, and you'd be right but lobbyists from these parking companies have pressured the government to ensure that legislation was changed to permit this as well as affording them, as private companies, access to DVLA records, which is again an obvious GDPR breach.
Ultimately you're going to have to defend your position as you would in any legal dispute, so be clear, send your evidence and keep records of what was sent and when, and what was received and when. In most cases (excluding ParkingEye) you will eventually receive a response, which may be auto-generated or may have actually been written by a human and in some cases this will dismiss the charge, but most parking companies will push and push on the basis that most people will give up considering it to be too time consuming. Stand your ground if you are in the right and keep pushing back by sending the same evidence in response to each automated reply. Eventually a human will have to make a decision at the company as to whether its worth taking you to court over the invoice, and at this point most will give up, not that they'll have the courtesy to tell you. If you do wind up with a summons to court (and not just one of their threatening letters) then you'll need to prepare for court. You don't need a solicitor but if you instruct one you'll be unlikely to get their charges back unless you can show that the parking company acted in bad faith. Remember You have the evidence and its THEIR job to prove that you were in the wrong.
There are a number of forums and companies on the internet who claim to handle to your case for you, but beware that these may not work in your best interest and you may wind up incurring more charges even if the case is subsequently dismissed.