Suffering occasional system freezes

[Moltke]

Hi [member=7114]ian_r_h[/member] an

My question is that when it comes to updating, presumably I need to boot from the current kernel (4.4.0-102-generic), rather than booting from an earlier kernel?

  There's no need to change the kernel LL boots by default, at least I have never needed to not in LL nor in any other distro I've tried.

Next time your system freezes, hope it doesn't but if it does try the following, press "Ctrl+Alt+F1" this will take you into a command line mode, give it a couple  of seconds and then press "Ctrl+Alt+F7" which will take you back to your desktop. If it works and your system is no longer froze then we can think of possibles causes.
You could also try a newer kernel version. Also, would you mind doing a system report with LL tool for that and attaching it here so people can take a look at it and help in diagnosing your issue.
Hope this helps!