The undated letters addressed to “The Occupier”, were pushed through doors along a cul-de-sac in Aldershot, Hampshire, on a Thursday in November. Those who bothered to open what looked like junk mail discovered that part of the road had been sold to a private company and they would have to buy a £2.50-a-week permit to leave their cars outside their homes where free parking had been available for 50 years. They also had to pay a £75 deposit or face a £60 further charge.

One resident, Elizabeth Thomson, was caught unawares, having been away. “I came back on Sunday night in the dark and rain and parked where I have for the past 10 years,” she says. “I woke up the next morning to a £60 parking ticket, and, in the daylight, I could see two signs warning of charges which had apparently gone up on Friday evening.

“Most of my neighbours were upset because they’d been fined on Saturday morning. The letter had made no mention of when the charges would come into force, and they were imposed two weeks before the parking bays had been marked out, and less than two days after the letters were received, giving insufficient time to apply for a permit. Overnight, our road was turned into a private car park.”

The residents’ plight exposes the injustices of unregulated parking enforcement which controls parking on private land, including supermarkets, stations and hospitals.

There are no rules on how much notice a company must give before imposing fees and restrictions. Although legally private parking enforcers are not allowed to issue fines or penalties, they can charge for costs incurred when their terms and conditions are breached. Many demand three-figure sums for minor breaches of their terms.

The £1.5bn industry earns most of its revenue from these charges, which are supposed to deter motorists from abusing the system, but which are increasingly imposed on those who have paid in good faith but fallen foul of a minor technicality. Last year, the number rose 26% to 5.5m, fuelled by automated number plate recognition (ANPR) cameras which record vehicles entering and leaving car parks.

Campaigners claim inadequate signs, unreasonable terms and exorbitant demands by some operators exploit motorists, and simple measures such as automated barriers and ticket machines synchronised with cameras, are avoided to boost revenue.

The Observer has featured the cases of drivers charged for confusing the letter O and zero when keying their number plate into a ticket machine. Others have been threatened with court action for calculating the length of their stay from the time stamped on the ticket, rather than the time cameras clocked them entering.

The Aldershot residents are likely to be among many who are suddenly faced with punitive costs as housing associations sell off garages and parking areas to investment companies, and private companies are hired to enforce new restrictions.

Thomson lives on a former council estate of roads and cul-de-sacs built in the 1960s and sold to a housing association 30 years later. They have now discovered that only part of each cul-de-sac had been adopted by the council. The sections at the end, containing homes and garage blocks, remained in the ownership of Vivid Housing Association.

Last year, Vivid sold off the garages and adjoining land to Conshurst Investments, which introduced the parking restrictions, including bollards to prevent residents reaching their driveways. One blue-badge holder now has to leave his car a five-minute walk away from his home.

Vivid says it sold the land to fund more affordable housing and was only obliged to notify those who rented the garages. Stephen Davies, director of Conshurst, says Park Direct was contracted to enforce new controls, and claims Conshurst manages the site well, preventing fly tipping or dumping of vehicles. But he confirms Park Direct receives no payment from Conshurst for its services or any part of the permit fee, adding: “I can’t see how they could earn anything else other than levying charges.”

Park Direct has not responded to our requests for a comment.

Punitive ticketing has soared since wheel-clamping was outlawed on private land six years ago. The Protection of Freedoms Act 2012 allows private parking firms in England and Wales to pursue the registered keeper of a vehicle for unpaid charges, regardless of whether they were the driver. They can obtain addresses from the DVLA provided they signed up to one of the two accredited trade associations, the BPA and the IPC. Both require members to abide by a code of practice and to cooperate with their independent appeals process.

Critics question their impartiality, given they are funded by member companies.

In February, a private member’s bill calling for statutory regulation of the industry received its second reading. A similar bill is being proposed in Scotland. Last year, the Society of Chief Officers of Trading Standards in Scotland inspected 71 car parks and found one in three were in breach of trading law.

Meanwhile, motorists have to take pot luck with the appeals schemes. The IPC has instructed members and appeals adjudicators to overturn charges issued when a zero is confused with an 0, as the mistake is too trifling to constitute a breach of terms and conditions. Popla – the appeals process operated by the BPA – however, has found against many who have made the same mistake. It says it’s unable to allow an appeal based solely on mitigating circumstances, even if it’s clear that the driver paid.

The last resort is legal action and a court may decide a charge for a typo is unreasonable under the terms of the Consumer Rights Act. The Aldershot residents are on shakier ground, according to Sarah Garner, a solicitor at DAS Law: “An owner of land has the right to determine parking restrictions,” she says. “As no contract was previously in existence, reasonable notice would not have to be provided to change existing parking terms.

“However, to be bound by the terms of a contract, an individual would have be aware of what they were agreeing and, if residents had not opened the letters, and there was no signage, they may be able to argue they were not legally bound to comply.”

Thomson, now told she owes £160, has refused to pay on principle. “I can’t believe that roads that have been used as roads for the past 50 years can be converted overnight into private car parks and it be accepted as legal,” she says.

When it pays to appeal

Bob Hawkes’s refusal to pay an unfair parking charge notice was vindicated when Parking Eye, Britain’s biggest private parking firm, took legal action against him and had its case dismissed by a court.

Hawkes had parked at a pay-on-exit car park at the Royal Free Hospital in London when taking his wife to A&E and paid the £3 demanded after entering his car registration in the ticket machine. “A week later I received a demand for £100 for underpayment,” he says.

Popla, the independent appeals service, upheld the charge when he complained. When Hawkes refused to pay, Parking Eye – which earns an estimated £25m a year from charge notices – launched legal proceedings.

“My initial view was the machine was faulty because it had charged me for one hour rather than two,” Hawkes says. “I then discovered that the automated number plate recognition cameras that record vehicles entering and leaving were not linked to the payment machine.”

The court dismissed the case when Parking Eye’s solicitor was unable to prove the ticket machine had been functioning correctly.

Parking Eye says it operates an audited appeals process. “If a driver disagrees with our decision they have the option to appeal to Popla.”