Choose British Airways' 'flexible' ticket, with free destination and date changes, and you could pay almost 17 times as much as you would for a standard ticket.
A BA flight from London Heathrow to Alicante in February cost £57 when we looked in January but ticking 'Flexible ticket - change your flight with no booking fee' sent that soaring to £966 for an 'Economy Plus Flex' ticket.
Yet BA's current 'Book With Confidence' offer means that the standard £57 ticket is already flexible. The airline is currently allowing customers to change their flight dates for free right up until the departure gate closes. At the end of January they extended the date you can use the vouchers provided up to April 2023, even for vouchers that have previously been issued.
Given current levels of travel uncertainty, most airlines have a similar scheme but, without delving into their terms and conditions, it's not always clear what the rules are.
And while some airlines, like BA, will allow you to change until almost the last minute, others will still charge you a fee unless you make the change weeks before your trip. Most airlines still offer 'flex' tickets - despite the fact that there may be no point in buying them, because standard tickets are flexible for many dates anyway.
Where fee-free changes are not allowed it can sometimes be cheaper to buy an entirely new ticket than paying to change the dates.
Book a flight departing up until 30 April 2022 and you can later move it to any new date until 30 April 2023 for free. You can also change the destination.
Changes can be made up until the departure gate closes (usually an hour before departure).
The only catch is that you'll be provided with a voucher in exchange for your original ticket. If your new flight is more expensive you'll have to pay the difference.
British Airways also sells Economy Plus Flex tickets, which include increased baggage allowance and other extras as well. Don't choose this option. You can buy all the extras more cheaply without it. We asked why it's continuing to sell the unnecessary and expensive 'flex' tickets and it didn't respond directly but did say: 'We offer our customers a wide range of options to give greater choice and to best suit their individual travel plans.'
There are currently no fees on new bookings for changes to date or destination. Unfortunately you can only make changes up to 14 days before travel. After that you'll have to pay a fee.
EasyJet also sells 'flexi' fares, which were between two and three times more expensive than standard fares when we checked. They only allow you to change your dates by one day before or after and you can't change the destination at all. They also include other extras but, again, it's cheaper to buy them separately.
Jet2's alone among the airlines we looked at in not offering free flight changes or selling 'flex' tickets. However it only costs £35 per booking to change flight dates and you can do it up to five hours before your flight.
That's probably more helpful, particularly for a group booking, than being able to change your flights for free but having to give several weeks notice, as you do with some airlines.
Ryanair is offering free changes for bookings made up to 30 June 2021. You can move your flight to any date up until 31 October 2021, and change the destination, as long as you do it at least seven days before departure. Customers are allowed up to two changes in that period before having to pay fees.
Flights bought in the u20ac5 flights sale promotion on 1 and 2 September aren't eligible.
Ryanair also sells 'flexi plus' tickets, which were between three and four times more expensive than standard tickets when we looked. They only allow you to change your date by one day, before or after your original flight. They also include other extras, which it would be cheaper to buy separately.
Fee-free amendments can be made for any flight departing up until 30 April 2021. However changes can only be made until 21 days before departure.
With circumstances changing so rapidly during the pandemic this is a considerable drawback. Tui's explanation was that most customers buy flights as part of a package and that - in the event of lockdowns or changes of government advice - package holiday customers would be refunded anyway.
For travel by 31 Aug 2021 you can make up to two changes of dates and/or one change of names without a fee right up to departure. Dates can be moved until 31 December 2022. If the new fare is higher you'll have to pay the difference but this is waived if the difference is less than £60 for Economy, £120 for Premium or £350 for Upper.
Virgin Atlantic also sells expensive and unnecessary 'flexible' flights. A standard flight from London to New York that cost £319 became £2,031 when we ticked the 'flexible' box on its website.
It was unclear from its website what the terms and conditions of this ticket are, or how flexible it is. Virgin Atlantic has not responded to our request for clarification but did say: 'We are constantly evaluating how best to ensure our customers are kept up to date with the latest booking policies.'