Little update on my problem with UESocket.io not connecting to my server. I'm not really sure how I've got it set up as i'm basically fudging my server code together, but it seems that the way I was hosting my webpage may have something to do with it. I started getting connections to UE4 when I set up second socket.io 'instance' in my server code, listening on another port.
My original setup is a node server which acts as a CMS reading and writing to an SQLite database and serving up a webpage. I bolted on lots of other stuff like OSC and stuff - and originally I was running with XMLHTTPrequests rather than Socket.io - then I switched to socket.io for communication between the server and the served webpage-interface but when I tried to connect from UE4 through the same address and port I got nothing.
Don't know what I did, and I don't' really like running 2 ports (it seems clunky) but it's working now.