Finally I have tracked down my #Qutebrowser issue that only seemed to mess with #DuckDuckGo search results returning an 400 Bad Request
It seems in my config.py I had the following user agent line.

c.content.headers.user_agent = 'Mozilla/5.0 ({os_info}) QtWebEngine/{qt_version} qutebrowser/{qutebrowser_version} Chromium/67.0.1'

I changed it to the same as #Firefox was using and bingo it all works !

c.content.headers.user_agent = 'Mozilla/5.0 (X11; Linux x86_64; rv:131.0) Gecko/20100101 Firefox/131.0'

Who's a happy bunny now.

CC: @silverwizard@convenient.email