Yep, the client's primary DNS is pointing at the IP address of the DC. Coincidently, the error message has stopped appearing!
I'm not sure where i've applied it tbh, or how to change that.. I opened the policy manager, went to Domains>headquarters.webservuk>Group Policy Objects and created a new policy object there. Was that not right? In the box just to the right of that it shows "Security Filtering - The settings in this GPO can only apply to the following groups, users and computers: Authenticated users".
I must be doing something wrong!

I'm not sure where i've applied it tbh, or how to change that.. I opened the policy manager, went to Domains>headquarters.webservuk>Group Policy Objects and created a new policy object there. Was that not right? In the box just to the right of that it shows "Security Filtering - The settings in this GPO can only apply to the following groups, users and computers: Authenticated users".
I must be doing something wrong!