.gov.uk site wont work but .co.uk will

We have a site 

localcounciljobs.gov.uk

When going through the web profile for the LAN it doesn't work.

When we attempt this on the guest web profile it works.

I have allowed this through the web filter - added to custom categories (even though its not a blocked site through the rating anyway)

if we access localcounciljobs.co.uk (which is the same site) it works fine

But we need this gov site accessible

The error is net::ERR_CERT_AUTHORITY_INVALID

and we cannot advance - it just reloads the page.

Any ideas what im missing here?

works fine OFF the web profile

Thanks