My wife is booking flights for next March. Going to the Monarch site and using the search function it turned up a return flight for 35€ outbound from Alicante and 49€ return from Leeds.

All well, she decides to book them as they are cheap. Entering the details and lo and behold the price changes! While the return leg stays the same, the outward leg has jumped to 150€

So she cancels where she is up to and returns to the search page, again the flight shows up as 35€ but on trying to book it the price jumps to 150€

Monarch trying underhand practices to get bookings?

I've never seen that happen before. Monarch's site has always given me the headline price.

Airlines underhand! Never!Ryanair win honesty awards every year.
Sometimes it works out better to book outbound flight with one airline then inbound with another (that's if they use the same airports)

That's what she's done Dave, Monarch lose out as only one leg of the flight now booked with them, the other is now with Ryanair, which by chance is a better flight time. But knowing ryanair the time will change several times between now and then.

Before actually making a booking, it is always a good idea to remove cookies as the more you search for prices, the more chance that that particular site will change the prices originally quoted.

I think some of the airlines have got wise to customers removing cookies.
I'm convinced that some of them now log your IP address which, unless you use a proxy, is difficult to change.

Sad they force you to do it,as for Ryanair if it's the same day they quote you've had a result

The cookies thing is an urban myth. There are sites that have carried out tests to prove it.

Over the last year I have planned several group trips which I've discussed with friends while arranging travel. Each time one of us has seen a price and called someone else, the other person has gone to the site and seen exactly the same price. I've never seen anything to suggest this "rigging" is going on.

And how would it cope with Skyscanner?

Are there any links to concrete evidence, I'd be interested to look at them?

My brother in-law has an MBA at ESADE, the top business school in Spain

As part of the course, Easyjet came in and explained exactly how they rigged the price by using the IP address. So it is not a myth I'm afraid

Your brother-in-law should have asked why they use IP address when an office with 1000 PCs in it share a single IP address as far as all the sites on the Internet can tell.

IP addresses wouldn't work for this single reason. They'd have to use something else or at least a combination.

Unless you have a fixed IP address, which is rare with Movistar, you can easily change your IP address by resetting the router - there is usually a reset button or you can just pull the power supply lead out.

That's right Fred, although it does depend on the lease time set on the DHCP server that gave you the address. I am not sure what the DHCP lease is on Movistar.

But you point to another issue with the idea that Easyjet and other others use IP addresses as a means of identification. If you and I were neighbours (I'm a considerate neighbour, honestly :-)) and we both rebooted our routers on the same day, Easyjet would offer you my prices, and would offer me your prices.

There are so many potential issues with using IP addresses to identify people that sites just don't do it. It's why cookies came into existence.

I booked one way with Monarch in January this year - good price but then there was a charged for checking in online. (You can/could check in at the airport for free.)

Actually I think you will find you were charged because you selected a seat in advance which is the only way you get to check in online with Monarch. If you would have skipped the bit where you choose a seat you will not get charged or be able to check in online and as you rightly say you do not get charged any more for having a seat assigned at the check in even if it is a premium seat

My daughter had a similar problem with RA. She went onto their "bargains" site but when she tried to book the prices increased. She phoned them twice to complain about this and on each occasion when she raised this anomoly the operator went silent but kept the line open. Seems you either pay up or dont go!