Oops! A bit late with this reply, sorry Badders
There was a f/w update to v1.52.10 which I've flashed, but still the same problem. Doing a bit of research via various forums/google it appears the Surfboard 4100 and the BEFSX41 have 'issues' in not wanting to talk to each other!
I can get my SB4100 to be recognised by the BEFSX41 but it takes lots of combinations of CM reboots, router resets/Power offs/Factory defaults to achieve a solid connection, so I still have to play 'Hit and Miss' whenever the connection is lost from the CM to the router.