Having said that I wasn't going to install any more security updates because they break my sleep (from the Apple menu) ... well, read on.
At the same time that the latest security update appeared, there was also a Safari update. So I decided I would just do that, and guess what? It broke my sleep. I can't imagine what there can be in a Safari update that would break sleep, but anyway ....
So, what I did this time was, try and sleep from the Apple menu, wait for it to fail after 3 mins, whereupon it would restart itself. But on restarting I did a reset nvram (Cmd-Opt-P-R). Tried the sleep again and it failed and restarted. Did the reset nvram again. In total I had to perform 3 restarts and resetting nvram before sleep would work. Why 3, I have no idea.
Having determined that resetting nvram 3 times restores the sleep functionality, I bit the bullet and installed the security update. Restarted into the patcher, and installed all the post-install patches. To my surprise, after that, sleep worked without having to do any more resetting nvram.
So, I'm now not sure whether it was the security updates breaking my sleep previously or some other update e.g. Safari, that I was doing at the same time.
I still had to jump through some hoops this time though. Firstly it complained that the Night Shift patch had been overwritten so that needed reinstalling. Even after that I had no Night Shift tab in my Displays prefpane. What's more, my audio was messed up, I couldn't adjust the volume with the volume keys (prohibit sign) and the Audio Midi Controller app was empty. So I did a re-install all from the Patch Updater and everything was fine.