I had time to check everything today.
Everything is working fine now with remote viewing.
The previous 'fixed' IP address on the Slingbox Pro had changed.
All the details on the Router remained unchanged, but Port 5001 was not open to the changed IP address, hence no remote viewing.
The mystery is why the IP address had changed? The Slingbox had not been reset, or lost power neither had the Router. The IP address was not dynamically allocated by the Router. Nobody had access to the Slingbox configuration to change it manually.
Oh well, all working, so no worries.
Just worth checking the Slingbox IP address if this ever happens to you.