OK

Now I have installed SANE backends list on my computer (Synaptic tells me it's there). Could someone be kind enough as to tell me how to get hold of it so I can uncomment the backend sane-umax_pp (#umax_pp), please? Or am I completely mistaken... What shall I write on the command line, gksudo what or what else?

You may with check if the Scanner is recognized by your system.

Check if there is now a group "scanner" in your /etc/group file, in your output in an above post it is missing. Add your users to the scanner group.

Markus

Did not find any parport scanner
I am not part of any scanner group (there is no such in my /etc/group file)
Think I'm gonna call this whole thing off and get rid of the damn thing. New, working ones can be purchased for a minimum of cost.