Fixed Menu Bar Moves on Scroll

Hi,

Currently building my site at flavourfool.com and keep encountering a problem with the positioning of my navigation menu.

I would like the navigation menu to remain fixed when scrolling. I have set the position to ‘fixed’, but when scrolling down the page, after a certain point (perhaps the header?), it jumps to the left.

I have attempted to fix this with margins, but when moving the navbar right to centre it further down the page, the starting position is then also moved right.

Can anyone help? Or is my problem one to be taken up with the respective support of UberMenu (which I am using for the navigation) or Vellum (my theme)?

PS. I have also tried making the width of the navbar 100%, but this seems to conflict with the max-width of the page layout, which is set at 1100px. If this could be achieved, it would also be a perfectly acceptable workaround!

Thanks,

Andy