By Anthony Reuben
Business reporter, BBC News
Are we being asked questions we cannot answer?
You are in a beautiful Spanish square, it is mid-July and 90 degrees in the shade, you have finally reached the front of the queue for the cash machine, and then it asks you the key question.
Do you want to be charged in euros or pounds.
Choose the second option, and the bank will tell you exactly how many pounds your euro withdrawal will cost you, at its own exchange rate.
Choose the first, and your own bank's rate back in the UK will prevail - although you have, for all practical purposes, no way of knowing what it is.
"The consumer would need the mental arithmetic skills of a Cambridge maths professor to work out which option would provide them with the best deal," says Dominic Lindley, policy adviser at Which?.
According to Mark Bowerman from Apacs, which represents institutions that provide debit cards, there is one clear benefit. "It's useful for people who want to know there and then how much they're going to be charged," he points out.
But there is a further catch. Your bank at home will almost certainly levy a charge for letting you use a foreign cash machine, even if you have chosen to let the foreign bank do the conversion.
So the amount the machine says you will be charged could be as much as £4.50 less than what you will end up paying.
The process is called Dynamic Currency Conversion, and it is an example of the sort of lack of transparency that has led the Office of Fair Trading to launch its investigation into whether there really is such a thing as free banking.
It says its study will examine "whether the widespread provision of so-called free banking delivers sufficiently high levels of transparency and value for customers."
300 euros taken from ATM
Telebanco charged £204.48
NatWest added £4 charge
NatWest would have done it 28p cheaper
In an example transaction earlier this year, a NatWest Maestro card was used in a Telebanco ATM in Granada in Spain to withdraw 300 euros.
The customer opted to let the Spanish bank perform the conversion.
Telebanco converted the currency for £204.48 and NatWest made a £4 charge for using an overseas ATM.
NatWest was later asked exactly what exchange rate it would have given at the time of the transaction - and said that if the user had opted not to let the Spanish bank perform the conversion, the transaction would have turned out 28 pence cheaper.
NatWest assures its customers that while it is a "matter of choice for the customer, they can be assured that NatWest will provide an extremely competitive rate".
Unfortunately, there would have been no way to find this out at the time - and although in this instance the UK bank was cheaper, there is no guarantee that this would always be the case.
The advice from Dominic Lindley at Which? is: "If in doubt, it is normally best to accept the charge from your UK bank."
Fee or free?
Choosing who performs the currency conversion may not always seem to make a huge difference for the customer.
But for the banks, it can be a lucrative business.
In our example, NatWest's charge was 2.25% of the sterling amount, up to a maximum of £4.
But had you chosen to use NatWest's own exchange rate, you would have found it was 2.65% below the market rate, to cover handling costs and processing fees.
NatWest is far from alone in this practice; most other banks will do something similar.
At Barclays, you would be charged 2% with a minimum of £1.50 and maximum £4.50 for using an overseas ATM, unless it belongs to Barclays or one of its partners, with a 2.75% hit on the exchange rate.
Lloyds TSB also knocks the 2.75% conversion fee off the exchange rate it gives you - and will then make an additional charge of 1.5% of the amount with a minimum transaction charge of £2 and a maximum of £4.50.
HSBC, in turn, makes a 2.75% conversion fee and a transaction charge of 1.5% with a £1.75 minimum and no maximum.
There is, however, at least one exception. Nationwide's current account adds neither a handling fee nor an additional charge onto your transaction.