It took a while to figure this one out Messages in log:-

[sqagtres] OnlineThread: Error 435 bringing resource online.

[sqagtres] OnlineThread: ResUtilsStartResourceService failed (status 435)

[sqagtres] StartResourceService: Failed to start SQLAgent$Dev_Sleuth service.  CurrentState: 1

Solution search the registry for the name of the incorrect server:-

You should find it in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL.X\SQLServerAgent set the new value to NULL

see http://blogs.msdn.com/karthick_pk/archive/2009/01/14/unable-to-start-sqlserver-agent-resource-on-cluster-after-upgrading-to-9-00-3186-or-higher.aspx