First it was retail parks, then came station car parks. Now drivers who fill their car up at garages, or use the car wash are being hit with £100 demands from private parking companies for staying too long.

Both BP and Shell have been quietly signing up parking firms to install CCTV cameras at the entrances and exits to petrol stations, which are then used to enforce a maximum stay limit – which can be 30 minutes at a BP station and as little as 20 minutes at a Shell station. Fail to comply, and you’ll face £100 demands and the threat of debt collectors.

The problem is that few drivers are aware of these maximum stays that have been appearing at forecourts all over the country. They particularly catch out those who, after filling up, go on to other services such as the cafes, shops, and especially, car washes.

The restrictions have to be clearly signed at petrol stations, but drivers say they can be easily missed when negotiating busy sites or junctions – particularly after dark.

Gareth Hughes, who lives in south London, is a BP customer who fell foul of this policy. He was recently sent a letter threatening the debt recovery agents after he failed to pay a £100 parking charge demand by MET Parking Services – a private company contracted by BP. His crime? He exceeded the 30 minute maximum permitted stay at the busy BP garage on Mitcham Road in Croydon, south London.

“After I filled the car, I went in to pay. I decided while paying to also use the car wash, and having paid for that returned to the car. There were perhaps six cars in front of me waiting to use the wash. I certainly didn’t see any signs warning of the 30 minute limit,” he says

It later emerged that his wife, the car’s registered keeper, had been sent a previous unopened letter from MET, which had demanded the charge, reduced to £60 if paid within 14 days. He had been on the site for 47 minutes, it said. He says he paid the £100 fearing debt collectors might turn up at their house.

Last year, media trainer and journalist Guy Clapperton had a similar experience at the same Croydon BP forecourt – receiving the same £100 demand.

“When I arrived there was a bit of a queue for the petrol pump. There is a Marks & Spencer’s Simply Food shop on the premises, so I stopped to buy a few bits. There was quite a wait for the till. After I had vacuumed the car and waited to use the car wash I’d been on the site 42 minutes and was sent the same £100 demand.”

After a long battle in which he contacted BP to point out it was penalising its customers for using its services, he was eventually refunded, but he says BP wasn’t interested in his plight.

“How can it make sense to penalise people who spend £80 or more on your services. An allowance of 45 minutes would be far more reasonable,” he said.

When Money visited the petrol station in Croydon this week, BP staff admitted that they receive lots of similar complaints, and that it had become a problem at the site. The cashier said he believed BP was installing cameras at all fuel stations over the country, but thought those using the car wash got extra time.

“They did it because people were just parking on the site,” he said.

Facebook Twitter Pinterest Warning sign at the back of customer car parking at the BP service station in Croydon. Photograph: Miles Brignall/The Guardian

As drivers enter the busy Croydon forecourt, there is a sign warning of the 30 minute rule. But it could be easily missed, particularly if you are heading southbound. Two of the five drivers we approached at the time told us they were unaware of the restrictions, despite having driven past the sign seconds earlier.

“It’s designed to catch out people who don’t know it’s going on. I mean £100 – it’s ridiculous,” said a local worker who had just parked his van.

It is not the first time MET Parking Services has appeared in Money pages. A year ago it emerged it had divided up a car park it managed near Stansted airport and was issuing tickets to diners who ate in the wrong restaurant. Victims of the policy said it caught out the unwary.

A MET spokesman says of the BP garage: “The 30 minute maximum is clearly signposted throughout the site. We would encourage any motorist who believes they were issued with a charge notice incorrectly to appeal using our BPA audited appeals process. Motorists have the right to appeal to the independent appeals service, Popla, which is a free and easy to use service.”

On behalf of Hughes, Guardian Money sent his case to MET’s lawyers 10 days ago. It has not refunded him and insists he must appeal in writing.

BP, which operates over 1,200 fuel stations in the UK, said they were unable to respond to questions sent by Guardian Money.

However, this is by no means just a BP issue. In July, Money featured the case of a London reader who received a £100 demand from Euro Car Parks after they exceeded a 20 minute maximum stay at the Shell fuel station in Kilburn. They had been stuck in a long queue to use the car wash. Their initial appeal against the charge was turned down by Euro Car Parks, although the charge was quashed at second appeal.

This week the same parking firm sent Emma Ockendon a £100 demand after it said she had exceeded the maximum stay at the Shell garage on the outskirts of Bridgwater, Somerset.

Like the others she had faced a long wait to use the on-site car wash, and was furious that she was sent a demand for spending money with Shell.

Shell says the parking restrictions are introduced at sites with limited space to “make it fairer for everyone”. Customers staying longer to use a car wash should input their car details into the car wash or shop console to avoid receiving any fines, it said.

Parking ‘fines’ – what to do if you get one

Despite what these letters imply, private parking firms can’t fine drivers for overstaying at a garage or retail park. Instead they pursue motorists for “damages” that arise from you breaching its contract.

If you get such a letter you have choices. You can take the militant approach and tell the firm you believe its demand to be unfair, and that you won’t pay it. The parking firm would then have to decide whether it is prepared to take you to court, and to prove their “loss”. In most cases they won’t, but it does happen. At this point you would have to defend the action, with the aggravation that entails. If you ignore a court summons, you will lose by virtue of not turning up.

Another option is to send the equivalent of what you would have paid locally to park – in the Croydon area it typically costs £1.70 to park for an hour. Insist the payment is in full and final settlement, and demand a receipt.

MoneySavingExpert, which has a page devoted to this problem – including useful template letters – advises drivers to use the appeals process. The parking firms will generally decline your appeal. Popla will consider appeals over tickets issued by British Parking Association members and currently overturns about 52% of charges. There is nothing to stop you going through the appeals process and then adopting the above approach.

Parking expert Barrie Segal says the code of practice requires parking signs to be clear and placed where they can be seen. Common law also requires that they are unambiguous. “The fact that so many motorists have been caught out suggests that these requirements are not being complied with. It beggars belief that BP and Shell effectively penalise their customers for buying their petrol and goods in their forecourt shops. It behoves BP and Shell to ensure that their customers are not penalised. Drivers who feel that they have been unfairly fined should appeal explaining the reasons why the fine is unfair. They should also complain to BP and Shell.”