I am being stupid? I cannot find an icon or box to search the site. The closest I got was just now when I started to type the title of this post.
You can find the search box at the very top of the desktop site:
On mobile, you can touch the magnifying glass icon next to the "All Questions" text:
And in the Android app it's again located at the top of the question list:
As Alexandre Vaillancourt points out, we also have a help page with more information about using the search feature.