Nick, sorry for the errors in the post, now fixed.
The underscores before and after the session ID were an attempt to trigger the bold characters, but this didn't work in the code block.

Your final "control.txt" request seems correct.
I see a space after the session ID, but I checked in our logs and there is no space, so you issued the request correctly.
I confirm that the only issue here is that you had interrupted the response to the "create_session.txt" request several minutes before.
Instead, the connection has to be kept active and the "control.txt" request has to be issued on a parallel socket.

Having two sockets available is a requirement with Lightstreamer.
You can avoid this requirement (for most usage cases) if you use a WebSocket.
This is not shown in our quickstart example, but full instructions on the communication protocol can be found here.