gpg problem with NZ ubuntu repo

On an apt-get update or aptitude update I get:

W: GPG error: http://nz.archive.ubuntu.com intrepid-updates Release:
The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu
Archive Automatic Signing Key <ftpmaster <at> ubuntu.com>
W: You may want to run apt-get update to correct these problems

anyone else seeing this or able to tell me what to do to correct it?

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