[MH Foodmagazine] Positioning MailChimp form in header section

I am trying to position a MailChimp signup form in the header section of my page, or to be more precise on the right hand side of the page banner, so that it is horizontally aligned with the right sidebar of the main content area.

If I use absolute positioning, it appears in the right place as long as the browser window is full-screen. As soon as I reduce its width, the form starts overlapping the logo, rather than staying to the right of it. However, if I use relative positioning, the form disappears underneath the menu bar and the banner, even if I give it a very high z-index value (I tried 99999).

Can someone explain to me what I am doing wrong, and what the best solution would be? The page is located at https://www.baguazhang.co.uk.

Thanks in advance for any help.