Of course technically you shouldn't change the master socket yourself. However genuine Openreach NTE5a sockets are available (plenty on eBay) and provided you do a neat job of installing it (get it on the wall straight, use the cable retainer zip-tie thing, don't short the incoming pair during the changeover) no-one will ever be any the wiser. It's only a case of a couple of screws to hold it to the wall and connecting two cores from the drop wire to the main A and B terminals at the rear of the socket (carefully fold any surplus pairs out of the way, don't cut them back).
Get a cheapy IDC punch down tool for connecting your new internal wiring to the extension connections on the faceplate and you are good to go.
As for "certified good enough for broadband" - unfotunately there is no onus on Openreach to provide a line that can support broadband, the only requirements they are beholden too relate to voice functionality.
Likelihood is if the BT remote testing is not detecting a fault and you pursue a fault investigation the line will come back as "right when tested" when the engineer attends and you'll be on the hook for the engineer visit charge.
Couldn't you temporarily (and carefully) disconnect your internal wiring from current the master socket and see if the noise is still present at the master socket itself? (you may need a punch down tool to reconnect it all again though!).
Edited by GeeTee (Sat 15-Mar-14 13:55:19)