The router will work BUT the problem is the User Name and Password are in the firmware so whilst you will be able to connect you will be connecting on the previous owners account. I've done this and it does work!
I viewed the Sagem as a standby replacement in case the original (valid) Netgear router failed. So I just confirmed it worked for a day or so.
It's not possible to change the User name and Password as these are permanently burned into the firmware. The only thing you can do is try to flash the firmware back to the original Sagem router firmware before it was loaded with Sky firmware. You can do this with the Netgear however I don't know about the Sagem. However you then finish up with a non-Sky router and Sky's T&V's say you must use their router on their service (if you don't they theoretically can disconnect you but more importantly if you have any problems they will decline to help).