Scottish Power, the clue is in the title: how can a pre-pay user owe you £345?

When I look back on this year, it will be interesting to see which companies have created the most problems for readers. Scottish Power is up there and yet again I have had to approach it on behalf of a reader.

In fact Garry Collingwood got in touch on behalf of his daughter Charlotte. "She's a single mum and was previously on a pre-payment meter for both gas and electricity," he told me. "As you are aware, the tariffs that all suppliers charge for this method of payment are nothing short of robbery with violence."

Garry overstates the case a little, but it's true that those on pre-pay meters are charged more for their energy – a system that penalises the low-income households who are least likely to be able to afford to heat their homes adequately.

Pre-pay customers are offered a limited number of tariffs and end up paying an average of £80 more a year for their energy bills than direct debit customers.

Back to Charlotte. She sensibly transferred to a credit meter with Scottish Power some weeks ago and immediately set up a direct debit to cover the cost of the energy she would use.

Then she got a shock. Scottish Power claimed she owed them £345.21 for past usage. She rightly complained and pointed out that she had been forced to pay for her gas and electricity in advance and so could not have run up such a debt.

The response from a slightly deluded Scottish Power worker was a classic case of self-inflicted confusion: "I have manually calculated the balance on your account and all is correct, it seems most of the balance on the account was transferred from the pre-payment meters."

Read that last sentence back. Common sense should tell you the only possible balance that could be transferred from a pre-payment meter would be a credit one.

Not according to Scottish Power. Rather than using some common sense, it was keen to encourage Charlotte to "arrange a payment plan to spread to [sic] balance from 12 to 60 months".

But common sense did prevail when I intervened and contacted head office on her behalf. After investigating the complaint, the debt was wiped off and Charlotte handed £75 compensation.

A Scottish Power spokesman said: "We apologise to Miss Collingwood for the inconvenience this matter has caused. We have carried out a full investigation and found that the start reads for both the gas and electricity had been estimated.

"The account has now been set up with current reads and the direct debit will be set up from December 2015," he added. "In way of an apology, we have applied a goodwill payment of £75 to her account."

I have accepted that the bulk of problems experienced by Scottish Power customers have been an unfortunate side-effect of the energy giant's attempt to improve its customer service operations in the past couple of years. But Charlotte's woeful story demonstrates that it can still manage to create an upsetting cock-up entirely on its own.