One section of homepage still overlapping Top Bar despite lower z-index

Hi all,

This is the first site I've built with GeneratePress and it's been excellent so far!

I've been trying to make the top bar sticky on my site, which I've now done and it works fine. However, there is still one section of the homepage that overlaps the top bar when scrolling. I'm not sure why this is happening because it has a lower z-index than the top bar, but I'm sure it's something obvious that I'm missing.

The site is https://staging.pitterselfdrive.co.uk/

Any help would be greatly appreciated.

Thanks,

Tom