I must admit to being an idiot, I did not check the Elasticsearch nodes because I assumed this was a web server error. AND of course there’s the error.
Caught exception while handling client http traffic, closing connection [id: 0xc240ea17, /xxx.xxx.xxx.xxx:14692 => /xxx.xxx.xxx.xxx:10102]
org.jboss.netty.handler.codec.frame.TooLongFrameException: An HTTP line is larger than 4096 bytes.
And if I then take a few seconds to google the error I find that I can configure this http.max_initial_line_length to fix my problem.
Testing that now to see if that does indeed fix it, and what I need to set it to.
Sorry to waste your time with inadequate troubleshooting!!