Not much I'm afraid, and since this is primarily a US site, the fact that you're on the other side of the pond means most people are going to be unfamiliar with your country's particular legal system.
Based on your grammatical skill, I'm going to take the leap and assume you're English, or part of some current/former English colony/commonwealth. So odds are your legal system will be based on the English legal system just like the US legal system, and will share a number of similarities. So basically your only real option would be to report them to any sort of government business watchdog organization, and possibly a private non-profit organization as well. Usually these groups are devoid of any real power to do anything, but you can file a complaint with them that other people could potentially look up.
You can also try taking them to your country's equivalent of small claims court. Here in the States, that's for anything up to about $5,000... So for the sake of math, let's just assume a 1:1 exchange rate with the Euro. You could attempt to sue, and have the court force Dell to figure out how much of the price they charge is for the warranty, and then refund you the balance of that final year. Probably all of which, and then some, would be eaten by administrative costs, so it would basically be a paper victory only.
In pretty much any country, wantonly lying to a court is an EXTREMELY serious crime, and any company caught doing it will be severely punished by the court. So if you could find some kind of evidence that they offered a 3-year warranty when you bought the monitor, and then filed papers with the court claiming it was never 3-years but only 2, then Dell's European unit would be in serious legal trouble. Of course you wouldn't see any of the massive fines that would likely be imposed, they'd go directly to the state and/or EU.
So the long and short of it is that there's really nothing you can do. But if you identify your country of origin, there might be someone here who's more familiar with your different options.
I know when I brought the Dell monitor direct from Alienware http://accessories.euro.dell.com/sna/productdetail.aspx?c=uk&l=en&s=dhs&cs=ukdhs1&sku=147262 from Alienware it had 3 years warranty started so on there website at the time when I brought but I have no prove.Alienware are saying it was only two years and I have seen the same monitor still being sold by other retailers still has the three years warranty in fact I never heard of a two years warranty always 3 years or 1 year.I contacted Dell gave them the details S/N Alienware Order Number but they could not find it on there syslem.So what are my rights is there anything I can do about this.
Power Button on Monitor defective is stuck and have to turn off monitor at the mains.