@ Orainsear,
I think your description of the screen bonding to the Faraday cage is correct but all modem PSUs I've come across whether 2 or 3 mains pins and including the 2 Wire one, only have a two conductor plug for the modem. On the 2Wire modem the Faraday cage is connected to the cable screens, the outer DC power pin and the mains earth pin. The connection diagram shows is a negative ground. However the only other PSU I have available from a Netgear DG834V4 is a transformer type with the same negative earth symbol diagram but the metal mains earth pin is NOT connected to the DC plug outer pin. That doesnt really matter, except perhaps to the standards inspectorates, as the modem has an all plastic case.
It follows that anybody with several modems and PSUs should ensure they are not swopped even if they are the same voltage.
I have also noted that both RJ 11 plugs have the drain wire soldered to the metal shroud so that could potentially produce an screen/ground loop IF both devices have shrouded RJ 11 sockets.
However I suspect all UK filter faceplates and filter dongles have plastic sockets rather than shrouded ones.
Just for completness the cable that L-Com use is 4 core 26 AWG Cat5e FTP into 6 pin RJ 11s with the innermost uncrossed pair using the blue pair, the next using the uncrossed orange pair and the two outer pins left unconnected.
Kind regards,
Walter