David, from Leigh in Essex, had the same thought when he had a similar call from Santander following an unexpected payment he had made by cheque.

BBC: Is it really your bank calling?

The structure of the charges can vary depending on payment method (by direct debit, pre-payment meter, or credit transfer such as cheque), on whether it is an internet-only tariff, which part of the country the customer lives in, if the deal involves a fixed price, when the deal was launched, how long it lasts, and so on.

BBC: Ed Davey announces 'four core tariff' plan

Consumers wanting to buy goods by mail order may be unable to pay by cheque because the seller will be unwilling to accept a payment that carries a levy of nearly 25%.

ECONOMIST: Borders and barriers