Bank of Scotland has refused to reveal how many customers are receiving a £56 windfall after failing to warn them of a rise in account charges.

The Edinburgh institution declined to give figures, a week after The Herald reported it was paying out to holders of its popular Ultimate Reward account.

Customers are being refunded the £2.50 increase in the account charge since it was introduced in September 2011, plus two months in lieu of notice and interest at 8%.

A spokesman for the Bank of Scotland said: "The vast majority of customers received a letter from us. A small, single-digit percentage was not sent a letter notifying them of changes to the cost of their account.

"If any affected customer is unhappy with these changes, they can close or change their account for free."

Andrew Hagger at information group Moneycomms said: "Although the bank is holding its hands up and doing the right thing by reimbursing those affected, it's worrying that this sort of large-scale oversight happened in the first place.

"Such errors highlight the importance of keeping a close eye on your bank statement."

He said when opening all new fee-paying or "packaged" accounts from last week, banks must ensure customers are eligible to claim for any insurance products in the deal and sales staff must point out features in the package that are not appropriate.