I don't think this is related to helping Brian with his query about why his
laptop won't sleep when he shuts its lid.
Getting back on topic, I reported it as a Debian bug:
Unfortunately, while I am clearly affected by this bug - which looks like a kernel bug to me with a work around in latest versions of systemd, it doesn't explain why suspend won't work after a reboot.
I also found I can manually force suspend without sudo:
systemctl suspend
Still, might be worth trying the latest version of systemd and seeing if it does help or not. At the very least I might get more debugging information.