Okay so I went to the Apple Store and said I wanted a replacement which they did. Opened it up in the store, quickly set up an account and still the same problem. Handed it back, said I wanted another one which they did...BUT...they said they are running out of this model so wouldn't be able to offer another until a new delivery came in. I said I would take it home and if the problem still persists I would leave it for a week to see if by chance the gradient went away...they mentioned the glue story!
Got it home, quickly set up an account and BOOM, there it is! Still the same issue. On another note I checked 2 machines on display and they also had the same problems.
What I would like to say is that the problem is very mild in comparison to the older models. You do have to look closely to see it but it's definitely there. I'm not suggesting that someone who has the new model has not got the issue but I've now checked 5 machines and all have suffered with it. If you have a new model and think it's not there it may be because the last models were so bad it was incredibly easy to spot but this one is vastly improved.
I find the easiest way to check is either create a white blank canvas in Photoshop and then drag the canvas over to the right and then slowly drag it back to the left and when you look closely you'll see a curtain effect where the left is suddenly not as white as the right. You can do the same with your email as well, drag it from the right to the left and you should notice it.
As I said it's so much better than the last models and the colour of the display is decent, not like that horrible yellow that was supposed to be white! Apple have clearly done something but it's still not 100%.
I'm not sure what to do now as I love the performance of these machines. Depending on how it goes in the next week I may take it back again but I've kind of got to the stage where I'm sick and tired of going backwards and forwards and not only that but it seems to be taking up a big chunk of my life so I may just accept that's how the screens are!