OK so finally got round to solving this one and it was the firewall in the end, WS(S) are not forwarded by default and have to be added/enabled to the Path-specific Routing rule:
Of course anyone else experiencing this issue may have a different firewall solution/setup so have to adapt to their setup.
In this instance it is a Sophos solution so the relevant info came via this post:
https://community.sophos.com/sophos-xg-firewall/f/discussions/84592/when-will-websocket-be-supported/387726#387726
The fact that WebSockets forwarding wasn’t enabled on the server in the first place wouldn’t have helped but I got there in the end.