this is covered in the FAQ
https://community.bt.com/t5/Home-phone-including-Digital/bd-p/Faults
The oblong sockets aren't RJ11 they are BT431. Simply connect a 431-431 double ended cord from the green socket to the socket in the office. You will need to use a doubler if you wish to retain a phone in that socket. You will need to isolate your internal phone wiring from the incoming line . To do this, fit a new extension socket adjacent to the current master in the hall and transfer the wires from terminals 2&5 in the master to terminals 2&5 in the new socket.
Thanks correcting the oblong socket type, as i was a bit confused by images online that showed both the square and oblong connectors as both RJ11. Also thanks for letting me know to use a 431-431 cord, I did wonder if something like that was available and would effectively re-route signals in reverse through my wiring system. A friend in the next road is about to be changed to VoIP and I'm sure my turn will come soon.
Thank you very much