By Simon Atkinson
Business reporter, BBC News
Hundreds of people each week are turning to 'no win, no fee' firms to get back penalty charges levied by banks and building societies.
Thousands are hoping their bank will be writing them a cheque
Claims handlers are typically keeping about 25% of any money won back.
Consumer groups and a body representing banks have told the BBC there is no need for intermediaries to be brought in to resolve disputes over charges.
But the firms argue they provide a service for those who are not confident in acting for themselves.
Banks have seen a surge in people trying to reclaim penalty charges they have been given on bank accounts and credit cards - often over many years - for going overdrawn.
Consumer groups such as Which? as well as the BBC are offering guidelines and template letters, assisting people to claim back the charges themselves.
One of the companies advertising its ability to reclaim bank charges on a "no win, no fee" basis said it was receiving about 150 inquiries a week - simply by advertising in one free London newspaper.
Phoenix Financial Recovery takes a 25% cut of any money which the bank returned - although it says it charges nothing if it does not win back at least 75% of the amount owed.
Managing director Philip MacDonald said that many customers preferred somebody else to handle the claims process - which involves writing a series of letters, as well as filing documents with a small claims court.
"If you are confident and have that kind of personality, then you can win it back yourself," Mr MacDonald said.
"However we think there's a reasonable niche market of people who don't want to have to issue a court summons or deal with it, and would rather have someone to hold their hand through the process."
He said that while cases very rarely went to court, there was a risk this could happen if forms were filled out incorrectly.
Banks routinely tried to cut their losses by making smaller offers, he added, which might tempt some people who feared holding out would leave them worse off.
Mr MacDonald added that banks also tried to find faults with applications, so that cases could be thrown out.
The company also worked out exactly the amount of interest that the bank owed on top of fees, he said.
A string of other companies are advertising similar services in newspapers and on the internet.
A spokeswoman for consumer body Which? said that while it was tempting to use a firm to claim back the money, would-be claimants should apply themselves.
"There's no reason to use a claim handling firm for bank charges," she said.
"They have no extra powers or persuasive abilities and we've heard of some firms taking between 20% and 40% as fees.
"If you can do the process yourself, why give away almost half of your money?
"We understand it might be daunting for some people, which is why we've produced template letters. All you have to do is fill in the details and be patient."
The British Bankers Association (BBA) was sceptical about the need for such firms.
"The best thing for a customer to do if they have a complaint or query about fees is to get in touch with the bank themselves," said the BBA's director of retail PR Lesley McLeod.
"There's no need for an intermediary."
Last week, the BBC reported that about 1,000 people a day were phoning the Financial Ombudsman to complain about bank penalty charges - 10 times more calls than it was getting a year ago.
In May, the Office of Fair Trading (OFT) said credit card default charges should reflect only company costs.
To the dismay of banks, the OFT added that this principle would apply to default charges on overdrafts.
In response, banking groups have said that unauthorised overdraft fees are a charge for a service - and that in some European countries, it is illegal for customers to go into the red without their bank's permission.