Anybody any experience of this, (or similar) happening? I assume the warranty is 12 months in the UK.
The manufacturer's warranty is 12 months.
The seller (that is the store where you bought, which may be Apple, which may be someone else) is responsible that the goods you bought are of reasonable quality. Half of the screen going dark within 18 months is not reasonable quality. Usually a computer should last at least for two years. The problem is that it is up to you to show that the problem is due to a fault that was present when you received the computer.
So go to the seller (Apple store if you bought it there, you can go to an Apple store if you bought from Apple online, or the store where you bought the Mac) and ask them to fix the problem, as the seller. Take a witness with you. If they say that the warranty is 12 months, you tell them that this has nothing to do with warranty, that they have to fix it because of the sales of goods act and because the goods were not of satisfactory quality. (To be clear, Apple doesn't have to fix it and won't fix it if you bought at PCWorld, for example. PCWorld has to).
If they say they are not going to fix it because it's out of warranty, you have won. You go to the small claims court with your witness. If they say they are not going to fix it because they don't think the problem is due to a fault that was present when the Mac was bought, then you have lost. You may have to find an expert examining your Mac and giving you an expert opinion that the fault was present at the date of the sale. You go back with that, they have to fix it, and refund the cost for the expert.
I read that Apple Store policy is to have their "genius" look at the product, and if he decides it was Apple's fault, they fix it. If he decides it's not, see above: Independent expert etc. etc. Other stores may have a policy of just saying "prove it". Again, see above. Having someone who can talk as if they know what they are doing is helpful. Making the person who deals with you _want_ to help you is _very_ helpful. Shouting usually doesn't help that much. Whether the fault was present when you received the iMac may very well be a matter of opinion of the genius, and whether he wants to help you or not may change that opinion.
Don't claim things that are not true. Like saying "European laws says you must give two years warranty". The shop will instantly not want to help you.
The "up to six years" that are mentioned are just the maximum time were everything runs out. Whatever happens, six years after you bought the shop can just say "we don't know you". Whether something has to last for six years wouldn't depend on how expensive it is, but would depend on the nature of the object. A wrought iron ornament that sits on a shelf and does nothing shouldn't spontaneously fall apart within six years, even if it only cost five pounds.