The ordeal began with a text informing Gerald Slater that he owed Scottish Power £300 and urged him to get in touch. Slater is not, and never has been, a Scottish Power customer and assumed it was a scam. Then the calls started. They repeated he owed £300 in unpaid fuel charges, although no bill had ever been sent to his home. The calls pursued him to his wife’s hospital bedside where she was being treated for cancer.

“At one point I had to leave the ward and broke down while talking to Scottish Power,” he says. “The representative did not even know why he was speaking to me as the call had been auto-dialled and he asked me to go through security to find out the reason.”

Slater was assured that his details did not match the indebted account and was referred to a complaints line. When he rang, he was told his complaint could not be accepted because he was not a Scottish Power customer.

Slater shares this onerous limbo with a crowd of other householders who are being pursued by Scottish Power for debts they don’t owe.

Pensioner Brenda Fuller* was hounded for eight years by letters addressed to a stranger demanding £7,000, although she has been supplied by British Gas for the 40 years she has lived at her home. The Energy Ombudsman upheld her complaint and ordered Scottish Power to cease all contact. Within months another £7,000 bill arrived with threats of bailiffs if she didn’t pay and she feared even leaving her house.

Paul Walsh was doorstepped by debt collectors acting for Scottish Power after he’d spent a year contesting bills for an account that wasn’t his.

Emma Gillard* and her partner tried for four years to persuade Scottish Power they had never been customers. The company continued to demand sums of up to £12,000 in letters addressed first to “The Occupier” and then, after they had identified themselves in their complaints, in each of their names. Eventually, it conceded an error, only to send debt collectors to their door six months later with a bill for £10,000.

All three only broke free when the Observer got involved. In all the cases Scottish Power blamed the national database which had linked the wrong meter numbers to their addresses. Meter numbers allow utilities companies to identify the households they supply, and it’s thought that as many as one in 10 are incorrectly recorded on the gas and electricity databases. Newly converted flats are particularly prone to mix-ups, leading to some people paying for their neighbours’ usage, and it’s up to suppliers to amend incorrect records. The trouble with Scottish Power is that it repeatedly fails to address the problem when bills are contested.

The utilities giant, rated “one star” by 96% of reviews on Trustpilot, was the most complained about energy supplier last year, with 8,441 cases referred to the Energy Ombudsman.

It appears to have learned little since it was fined £18m by the regulator Ofgem in 2016. Billing chaos caused by a new IT system prompted the penalty and Ofgem admonished it for inadequate complaints handling and unfair treatment of customers.

Four years on, mismatched meter numbers and abysmal record keeping is unleashing new havoc. Householders who try to contest phantom bills face being told there is no record of them on their system. If, like Gillard, they confirm their identity to customer services, they risk mystery bills addressed to “The Occupier” being resent in their name.

Slater, in desperation, fired off two letters to the chief executive. In response he received a bizarre email from the directors’ support team which appeared to confuse him with another customer. It thanked him for accepting a nonexistent phone call that morning and confirmed that his nonexistent account would be restored to a dual-fuel tariff “as requested”.

In vain, he has pointed out that he received no such call, made no such request and had never had an account with Scottish Power. The Energy Ombudsman told him it was unable to investigate because he had not completed Scottish Power’s complaints process. Scottish Power insists he can’t make a formal complaint because he does not have an account.

In the last month several more non-customers have contacted the Observer in similar despair.

Anita Franklin begged for help after Scottish Power representatives gained entry to her disabled son’s house and changed his meter to a prepayment unit while he was out. Her son has been a customer of Npower since 2011, but last September, out of the blue, a bill arrived from Scottish Power demanding £1,600.

“They say they have been supplying him for the last six years, but we have never been billed by them,” she says. “The bill is now nearly £2,000 and they’re claiming we have three accounts, one of them set up with someone else’s details. But we have no idea why, as no one ever answers our questions on the numerous times we’ve phoned and emailed.”

Dave Evans is being chased for £5,445 for electricity used between May and November 2017. He had moved out of his rented apartment nine months before the billed period and npower had supplied his energy while he was there. “They are requiring me to prove I wasn’t residing at the address during that period, and have stated that only information from the owner or landlord would help,” he says. “I have tried to contact my previous landlord via email, but this has bounced back.”

Guy Bull is trying to fend off demands for £895 in the name of “The Occupier” after Scottish Power decided he was a customer, even though he is paying another supplier. “When I contact Scottish Power they ask for my name and meter number but I don’t want to reveal them in case they attach it to the debt,” he explains. “I do not live in a flat, so I am not sure how they manage to make this mistake, but they certainly don’t seem to want to do anything about it.”

Ofgem said that this scale of misbilling seems to be peculiar to Scottish Power. “We are engaging with it on this type of issue, alongside wider issues with how they resolve all customer complaints,” it says. “We’re not aware of this being widespread among other suppliers.”

Scottish Power’s press office is as elusive as its customer services with departed officers listed as contacts, email addresses incommunicado and phone lines not answered. The Observer first alerted it to complaints in the new year. Still no response.

Slater has now sent a third letter to head office but received no reply. “I’ve had to put a block on Scottish Power calling my mobile whilst I am at work,” he says. “The months of harassment of myself and my sick wife for a debt we do not owe, and a complaints system apparently designed to cause maximum aggravation, is cruel and surely illegal. They are calling my good name into question and, at the very least, should change their chaotic systems to ensure my treatment does not happen to anyone else.”

* Names have been changed