This is what it looks like at full screen - this is OK.

When I reduce the screen size to just below the RWD value, it looks like this: note the large purple bar that is still there. This is the issue.

Then, on reducing it further, you can see it's layering over the top of other objects. If the large purple bar wasn't there, I wouldn't have this issue. Can anyone suggest a solution (and making the background alpha zero introduces other issues)

Any help appreciated.
Alan