Where the extension socket is only going to be used to plug in a router/modem the best way is to have a filtered faceplate in the master socket, connect a twisted pair (one pair of your CW1308 wire) to the xDSL (AB) connectors on the back and then at the other end have a straight unfiltered dedicated data socket in to which you plug the router directly.
Thanks for that confirmation, jelv.
My master socket is in the garage, and my modem/router is at the far end of an internal phone line that goes both upstairs & downstairs, with several outlets along its length. At least some of the sockets are daisy-chained (two cables to a socket), but I don't know if they all are. If you look at the latency graph (Westie) on MDWS, you will see it's not the cleanest of lines.
I'm due to change to FTTC in a week
and I've been concerned about noise from the internal wiring. I happen to have ~100m of BT 4-pair 0.5mm cable and I was considering running a separate line direct to the modem. I know it's not cat5e standard, but it should cope with 40Mbps shouldn't it?