Header problem

Firstly my apologies for this posting – I am pretty sure that I have seen the answer somewhere – but I cannot find it now after a very lengthy search.
In Responsive Shop 6.5, I have just uploaded my (max sixe) 1170 x 200 px header which I believed would fill the entire width of the fixed width page. But I am left with a vertical white bar to the left of the header image. Why is this and how do I get rid of it please? Image attached. Many thanks in advance for your help.