I had already included the startup call in my rc.local startup script, hence my ability to connect when I start the X11 session manually. Looking at the logs (copy attached) from the failed attempt to connect to the auto-started session it looks like things start going wrong some way into proceedings:
528 6136 09:04:59 050.141 ClientSession: Runnable at 0x443f540 caused the session at 0x227e6b0 to fail.
528 6136 09:04:59 050.141 ClientSession: Failing reason is ‘Could not connect to the server.
Error is 107: Connection refused’.
I don’t imagine the issue is too complex and some insight into what “Runnable at 0x443f540 caused the session at 0x227e6b0 to fail” actually means might help in the diagnosis.