I had to look up the meaning of idempotent, a most apt word if I may say so (pun not really intended but serendipitous anyway!). Your answers are very full and much appreciated.
>You can find preinst/postinst and other package tracking stuff under
I had one look at this directory and gave up the idea of trying to run these scripts, I think I'd be out of my depth. Reinstallation would be easier for me. However I don't have any reason to believe my system is faulty through a failure in updates so for the present I'll choose to be comforted by your belief that it hasn't caused any problems.
Thanks again for your help.