Hi, @MW0DCM-Dave (David Maydew) 
Regarding this part of the last paragraph of your post:
"(...) Update - Just after I made this thread the screen went black with a blinking cursor and wouldn't let me get to the Terminal, yet this is using 5.15 on 22.04LTS"
There are several reports, here in the Ubuntu MATE Community discussion forums, of computers that have an "Intel HD Graphics" card running Ubuntu MATE 22.04 LTS ("Jammy Jellyfish") where the graphical session "crashes" leading to a blank screen / black screen with a blinking cursor that then leads again to the Ubuntu MATE login screen. The good news is that in one of those discussions - Ubuntu Mate 22.04 auto ending Sessions when I do something specific - the user @esan_br (Elias Andrade), that has a "Dell Latitude E6410" with an "Intel HD Graphics" as its Graphics card, solved the issue that was happening to him (MATE crash that took him back to the login screen), in "Ubuntu MATE 22.04 LTS" ("Jammy Jellyfish") by:
1 - Creating a file "/etc/X11/xorg.conf.d/20-intel.conf"
2 - Putting the following content in that file:
Section "Device"
Identifier "Intel Graphics"
Driver "intel"
Option "TearFree" "true"
EndSection
3 - And then rebooting the computer.
Link to that post from @esan_br that he wrote on 1st October 2022:
In that same discussion, the user @Ygor6889 (Ygor Oliveira Silva), that also has a computer with an "Intel HD Graphics" video card, says that @esanbr's solution worked for him as well:
So, @MW0DCM-Dave : if your computer also has an "Intel HD Graphics" card, it may be worth a shot to create that "/etc/X11/xorg.conf.d/20-intel.conf" file with the content that I mentioned above to see if that, at least, solves that most recent issue / problem that you got of getting a black screen with a blinking cursor.
I hope this helps