http://www.amazon.co.uk/Rj45-Socket-To-431a-Plug/dp/... from a quick google
my search term was :-
RJ45 socket to BT plug
Your are on the ball "yarwel", thank you. I had previously seen the Amazon page but hadn't the confidence that it was an RJ45 and not RJ12 connector.
So I think my choice will be with http://www.run-it-direct.co.uk/bttorj45patchkit.html
but I bet it is a secondary connector for the 'remote' end when a master is actually required.
The combination of these two items allows a fax machine to be connected over the structured wiring, picking up the telephone service from a remote telephone socket.
A further use is in conjunction with a Vigor 3820Vn to bring the VoIP service to a POTS phone over the structured wiring.
I am at a loss why these useful bits to do this task are relatively difficult to track down, I would have thought there would be a healthy demand for them.