[DebianGIS] Unable to access repository

[David Pleydell <dpleydel@univ-fcomte.fr>]

Dear list users

I cannot successfully follow the instructions in the guide http://pkg-grass.alioth.debian.org/cgi-bin/wiki.pl/DebianGisRepository.

I've updated apt and downloaded the public key as described (without piping to sudo) but if I run 

apt-get update

I get the messages

W: GPG error: http://pkg-grass.alioth.debian.org testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C9954149FB2173F6
W: You may want to run apt-get update to correct these problems

What is unclear to me is that the documentaion talks of public key FB2173F6 but the error message talks of public key C9954149FB2173F6.  

How might I best overcome this apparent inability to use your repositories?

warm regards
David