It's known as vertical banding and can be common on cheaper LCD TVs where the backlights are poorly fitted.
Some TVs suffer more from it, others can have very good screen uniformity but it's a bit of a panel lottery as to which you'll get.
All you can do is contact Sky and explain the situation. If the banding is very noticeable and distracting to your viewing experience then you'll need to convince Sky that it is a defect of the unit. They may offer to replace it.
