The German words "Der Kater Isigeigei" keep popping up in my memory
and I can't place them. I think they are either the title (or part of
the title), the first few words and certainly the subject of a piece
of German writing - possibly a poem. I must have met them about 60 odd
years ago and they may (or may not) then have been ainchent. What is
the proper title, who is the author and where can I find the original
Geman text?

Dear nosin,

The proper name of the character is "Der Kater Hidigeigei".
Hidigeigei, a haughty green-eyed tomcat, is an accompanying figure in
Viktor von Scheffel's (16 February 1826 - 9 April 1886) verse epic
"Der Trompeter von Säckingen" ('The Trumpeter of Säckingen', 1853). As
Scheffel's organ, Hidigeigei accompanies the romance of the common
young man Franz Werner Kirchhofer and the noble-born Maria Ursula von
Schönau, who became in Scheffel's poem the character of Margaretha.
The time-critical and philosophic ideas of Hidigeigei are best
expressed in the thirteen "Lieder des Katers Hiddigeigei" ('Songs of
Kater Hidigeigei'), contained in the poem.
The complete work "Der Trompeter von Säckingen" is public domain now,
so I can provide you links to it as published online by Projekt
Gutenberg-DE:

The 13 "Lieder des Katers Hidigeigi"
http://www.gutenberg2000.de/scheffel/saekking/saek14b1.htm

The full text of "Der Trompeter von Säckingen"
http://www.gutenberg2000.de/scheffel/saekking/saekking.htm

Source: Projekt Gutenberg-DE
http://www.gutenberg2000.de/

You might want to find out more about the poet Viktor von Scheffel, so
I also located a short biography (in German):

Ilmenau.com: Joseph Viktor von Scheffel und Ilmenau
http://www.ilmenau.com/scheffel.asp

Additionally, here is the historic background of the "Trumpeter of
Säckingen", who was a real person, living in the 17th century:

Judo-Klib Bad Säckingen: Bad Säckingen - Die Geschichte des Trompeters
http://www.aocd.de/jck/regio/badsaeck/ordner/trompeter.htm


Additional source used:
Judo-Klib Bad Säckingen: Bad Säckingen - Kater Hiddigeigei
http://www.aocd.de/jck/regio/badsaeck/ordner/hidig.htm

Search terms used:
gedicht "der kater"
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=gedicht+%22der+kater%22&meta=
"Der Kater Hiddigeigei"
://www.google.de/search?q=%22Der+Kater+Hiddigeigei%22&hl=de&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&start=10&sa=N
"viktor von scheffel"
://www.google.de/search?q=%22viktor+von+scheffel%22&ie=UTF-8&oe=UTF-8&hl=de&meta=

Hope this answers your question!
Best regards,
Scriptor

Scriptor's answer is impressive: Not only has the researcher produced
the right answer from the faulty data supplied by me; as a bonus I got
historical notes, the whole text handed me on a plate and last but by
no means least, I was given an insight of how an experianced
researcher can get results in situations that stump me.