I would love to get hold of a copy of Ken Russell's film on Debussy,
which I think is actually called 'The Debussy Film' - Oliver Reed and
co. in it. I believe Debussy's descendants didn't like it and banned
anyone from reproducing it. Does anyone have an idea where I might
find a copy ?

I think the answer to your question probably lies in the following
story, which I will summarize, since the original material is
copyrighted. This narrative comes from the online memoirs of one of
the film's actors, "The Eyes and the Voice: The Memoirs of Vladek
Sheybal." They were originally intended for print publication;
however, the actor died in 1992 before he could complete a draft
manuscript. His estate has allowed the unfinished project to be
published on the internet.

In Chapter Twenty-One, which is devoted to memories of working with
Ken Russell, Sheybal relates the fascinating story of a second showing
of the Debussy film in 1988, 23 years after it was first screened in
1965 on Momento, a regular Sunday evening BBC Arts program during the
1960s. He received a phone call from the people planning the opening
of a new art centre (he later calls it a "club") in Paris. They wanted
his help in securing a copy of the film to show for the opening night,
and also wanted him to be there for the event to introduce the film.
Sheybal contacted Ken Russell, who said that the BBC owned the rights
to the film and would have it locked away in their archives if they
hadn?t already destroyed it. Using contact information he got from
Russell, Sheybal spent several days telephoning, finally getting a
conditional ?yes? from the BBC for the showing of the film, provided
it would not be shown commercially (the "club" was non-profit). In
addition, the network required the film to be accompanied by a BBC TV
security guard, whose expenses for the trip would have to be
completely covered by the Paris organization. Finally, the film could
be shown only once. All the conditions were met, and the film was
screened at the opening for only the second time in its history, a
fact that Sheybal laments as he recounts his perception of the
audience's entranced response. Presumably, it is still locked away
somewhere in the BBC vaults.

The issue with Debussy's estate is mentioned briefly in the write-up
on Russell's film on the website of the British Film Institute. You
can read the relevant comment in the first parapraph on this page:

http://www.screenonline.org.uk/tv/id/495009/index.html

Here is the link to Sheybal's memoirs. You will find some additional
comments about working with Russell on the movie in the chapter I
cited earlier.

The Eyes and the Voice: The Memoirs of Vladek Sheybal
http://admin.vladeksheybal.com/

Some additional, perhaps more hopeful, information. I've found
evidence to suggest that "The Debussy Film" has in recent years been
at least occasionally accessible to the public through special film
retrospectives and festivals. The movie seems to have been shown in
the U.S. as part of this event in late 2001/early 2002 (it is
mentioned in the schedule that follows the announcement below):

The Museum of Television & Radio Launches 
Global Visions, a Multipart Screening
Featuring the Work of International Directors 
 
Series to Include Alan Clarke, Stephen Frears, 
Mike Leigh, Ken Loach, Ken Russell, Ingmar Bergman, 
Rainer Werner Fassbinder, Ruy Guerra, and Tomás Gutiérrez
 
Part One, Disruptive Directors of British Television
October 18, 2001, to January 20, 2002, in New York
November 1, 2001 to January 20, 2002, in Los Angeles

http://mtr.inet7.com/PressRoom/pressRelease/10102001.htm

And it looks to have been screened during a European film festival as
recently as last summer:

http://www.filmfest-oldenburg.de/index_e.php?page=01a&news_ID2=185

Finally, to make a small correction to Mr. Sheybal's belief that the
1988 showing was the second time the film was screened to the public.
According to Ken Russell's own website, "The Debussy Film" was
repeated on the BBC in 1966.

http://www.kenrussellenterprises.com/career.htm