Hi arQon, thanks again for all the information and advice, here are my answers so far:
I haven't encrypted the home folder and AFAIK am not using encrypted swap, should I check for that?
In terms of the start up sequence , the cursor looks like an underscore in the top LH corner, but it doesn't blink. A log in dialog flashes up briefly but disappears before I can type anything. Then it's back to a black screen with no cursor for a bit, before the Ubuntu logo/wheel (this might be the boot splash?) appears with a progress bar underneath. That runs for a bit then more black screen until the main screen opens. During the whole time the hard drive light comes on briefly and intermittently and it churns away for a bit then stops and starts again. It's not grinding away constantly during the process, in fact most of the time nothing much seems to be happening.
I put sudo pluma /etc/default/grub into a terminal and got the following lines (there was more but this is the bit you were referring to):
GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=lsb_release -i -s 2> /dev/null || echo Debian
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""
I deleted "quiet splash" and left it with the two quotation marks as you suggested. I tried the commands to disable the services but the terminal wouldn't let me enter the password. The cursor blinked but I couldn't enter any text.
I'll reboot now and see if removing quiet splash has changed the process.