Thankyou peterwhite23. What you say makes sense, and i’ll bear it in mind.
For the moment, I’ve found what appears to be a reliable (?) way to boot up. From the Ubuntu boot offer, I select Advanced choices which offers 4 alternatives: normal and recovery mode for 2 different linux versions (I’ll try and copy it more exactly next time) - not distributions but maybe kernels?
If I go for the recovery mode in the older version, it ends with a box which I can’t completely read because it’s overlaid partly by another message - but I hit enter and it then displays a warning about possible display problems arriving and if so, then to use a standard restart to recover.
Sure enough, the display is crap - 1024x768 4:3 but I can see the restart button and that works.
I don’t know if that will continue to work, but I have new hardware on the way for next Monday or Tuesday so I’ll push my luck with this procedure until the weekend when I’ll give some of your suggestions a try.