Setup issues with mirroring (2008 r2)

Hi
I am trying to set up high performance mirroring between a primary server (ps1) and a secondary server (ss1)

When i tell the mirroring to start, the following error appears;

TITLE: Database Properties
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An error occurred while starting mirroring.

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ADDITIONAL INFORMATION:

Alter failed for Database 'Sue_Test'.  (Microsoft.SqlServer.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.4000.0+((KJ_PCU_Main).120628-0827+)&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Alter+Database&LinkId=20476

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An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)

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The server network address "TCP://<SS1<.<domain>.local:5022" 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. (Microsoft SQL Server, Error: 1418)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.4000&EvtSrc=MSSQLServer&EvtID=1418&LinkId=20476

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BUTTONS:

OK
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I have checked that the ports are enabled, checked that I can see the SS1 from PS1 and have checked that the account that starts the services on each side has permissions.

Please can anyone advise me what else I need to do?

Thanks
Sue