'apt-get update' GPG error

["alan bonard" <adam.debian@gmail.com>]

Hi, when using 'apt-get update' I keep getting

>Reading package lists... Done
>W: GPG error: http://www.mclean.net.nz stable Release: The following
>signatures couldn't be verified because the public key is not
>available: NO_PUBKEY 1AF1A20A4CC00851
>W: GPG error: http://www.debian-multimedia.org etch Release: The
>following signatures couldn't be verified because the public key is
>not available: NO_PUBKEY 07DC563D1F41B907
>W: GPG error: http://deb.grml.org grml-stable Release: The following
>signatures couldn't be verified because the public key is not
>available: NO_PUBKEY F61E2E7CECDEA787

...  which affects grml maintained

>Building dependency tree... Done
>The following packages have been kept back:
>  libvolume-id0 udev

... on which two xorg modules in ltsp5 depend.

Can anyone suggest a fix ?

Adam Bogacki,
afb@paradise.net.nz