SQL Server Mirroring with Certificates

I'm struggling with setting up mirroring with the following configuration:

• Server 1 running SQL Server Standard 2008 R2, Principal


• Server 2 running SQL Server Standard 2008 R2, Mirror


• Server 3 running SQL Server Express 2008 R2, Witness

I followed the tutorial here from Microsoft twice now to try and eliminate user error to no avail.

The issue that I am facing is that I get as far as setting the Principal as the partner to the Mirror like so which executes correctly and succeeds:

ALTER DATABASE Tools
    SET PARTNER = 'TCP://***:7022';
GO

I then go to set the Mirror as the partner to the Principal like so and get the error message The server network address “TCP://***:7022″ can not be reached or does not exist. Check the network address name and that the ports for the local and remote endpoints are operational.

The ports are definitely accessible and I've tested these with telnet with no issue (note the *** are a placeholder(!)).

I'm using certificates as per the above guide which are all setup correctly and each ENDPOINT is running as expected.

The only thing I can thing of is that the after restoring the full backup and transaction logs, the database on the mirror is still showing restoring but hangs like that for an extremely long time.

Is this reason for the Principal not accepting the Mirror because the Mirror has not finished restoring (I'd assume this is not the case as the restore was executed as NO RECOVERY) or am I missing something here?