I had this problem with a Philips and Currys which is outlined in the Currys bad experience thread. Though the backlight wasn't problematic in the corners like yours but randomly over the whole screen.
Would be interesting to know if that sort of backlighting issue where it appears only in the corners actually disappears after a certain period of time
Anyway whether it does or does not it's still a poor do that you should have that sort of rubbish on your screen after paying £1660 for a new TV.
Manufacturers should get their act together and stop churning out stuff like this if they are not able to manage making screens to an acceptable quality. I they can't then perhaps we should all go back to buying CRTs !
How local is a local dealer supposed to be

Perhaps a local independent dealer ? I only ask because ED is just a mile or so down the road from me.
I personally don't like them either, but they are an alternative to the likes of Currys & I did recently buy a home cinema system from them as they are local, were the cheapest and I was able to collect it rather than have it delivered.
You have to stand firm, say you previously owned an LCD which it replaces and did not have such a problem. And say it's not running to manufacturer's spec as there are lighter patches on darker scenes.
You have the right to return it under the 7 day distance selling regulation as the product is not satisfactory.
If they do not want to acknowledge this send it in writing to them stating that this is UK law which they are refusing to acknowledge and that you will be bound to seek legal action if they fail to refund or replace.