Q. A long time ago I booked a London-Rio return ticket on British Airways. Then I got a job that requires me to go to the US on a training course. Because of the dates involved, it made sense for me to ask the company to fly me straight to Rio – saving me from crossing the Atlantic twice. BA is now saying that I will not be allowed to take my return flight home if I don't show up for the original outbound flight. The cost of buying a new flight from Rio is £1,000. Michael, Hereford

A. You have been badly advised on two fronts: both by whoever arranged the US-Rio ticket, and by whoever said that a one-way flight from Rio to London costs £1,000.

British Airways, like many other "legacy" airlines, clings to the rule that if you fail to show up for the outbound leg of a return journey, then the entire trip is cancelled. The reason for this is bound up in the arcane structure of air fares, whereby many long-haul flights cost more for a one-way trip than a return journey. A good travel agent knows the potentially expensive consequences of being a "no-show".

Whoever arranged the US-Rio flight should have asked: how are you planning to get back from South America? Alarm bells would have started ringing if you then explained you were travelling on a BA return trip.

However, the cost of buying a new flight from Rio can be a lot less than £1,000, as long as you shop around. That fare is for a non-stop flight on BA, but if you are prepared to change en route, you can basically halve the cost.

The cheapest ticket is around £440 one-way on Ethiopian Airlines, though this is a long trip that involves a stop in Togo and an overnight stay in Addis Ababa. It would be better to make your way by bus to Sao Paulo, then fly with Air Europa to Madrid for about £425, and book a cheap one-way on easyJet, BA or Iberia onward to London. The total price should come to about £500.