Broadband issue, old UK wiring - any help available ?

I've been having problems with our ISP and BT not agreeing about a fault that is on our telephone line.
The problem is an unreliable internet connection, and the most obvious symptom is that if you lift our telephone socket you can hear the modem connecting. Whilst the internet is in use we get a great deal of static noise on the telephone line.
After extensive investigation and trying several pieces of equipment (different modems, cables, filters, etc) , it's become obvious that the problem is either with our internal wiring, or BT.
Given that BT charge exorbitant amounts if a problem proves to be internal; and our home was connected c.1970; I've decided to try and eliminate the internal problem before going any further. Incidentally, we have one master telephone socket - no other sockets or extensions.

I'll get the disclaimer in here first: Yes I take sole responsibility for any wiring and realise BT may get upset if I change their socket myself.

We have 4 wires coming into the home, connected to an existing std (2 part) BT socket as follows:
Orange -> A
White -> B
Green -> Unused
Black -> Unused

I have a new master socket which has 6 connections (numbered 1-6) and would like to use this new socket but all the information I have found refers to different colour combinations and I've been unable to find how to connect my existing wiring to this new socket.
I'd really appreciate any pointers as to which wires need to be connected up to which of the 6 terminals in the new master socket.