Re-installing the mutlaqja repository cures the problem. The error was somehow associated with the previous apt-add-repository although the repository information was correct i.e. identical to the information on another computer where apt update and install worked.
I failed to reload Ubuntu easily - the installation USB stick seems to detect the same rev of Ubuntu already running on the main drive and keeps deferring to it.
I loaded openSuse instead. It was a slow load but went in no bother, and I like the neat way the OS runs; it seems very complete and professional. The INDI instructions only went as far as installing a repository, but I found KSTARS in it, it installed easily and came up perfectly!
Now the challenge will be to find drivers that work for my telescope head. It's a Meade LXD75 and is not listed in KSTARS. I plan to try the LX200 drivers.
Does anyone have a source pack for the LXD drivers that I could build for openSUSE?