That's a standard adapter which will pass the four connections of the BT431 plug straight through and tap the A and B wires (tip & ring respectively) for the RJ11 jack. The line appears on the OUTSIDE contacts of the BT plug (terminals 2 and 5).
As Pat suggested, if you had a similar adapter before which had four contacts on the modem jack, then the outer contacts were almost certainly just spare.
There is nothing in the adapter/splitter itself which specifically makes the RJ11 a modem jack. It's just the standard configuration used in the U.S. for a single line, and as modems sold in the U.K. now also use an RJ11 configuration this adapter means that you can connect them with a straight modular cable.
In a simple domestic system white/orange (terminal 4) is superfluous, but orange/white (terminal 3) may be needed and should be connected.
When this jack system was introduced in the early 1980s, British phones were wired to accept ringing on that third wire (by way of the capacitor at the master jack). In more recent years, many phones (intended for world markets) have appeared which don't actually rely upon that third wire and will work with a basic two-wire connection.
By contrast, in the U.S. the standard connection is just two-wire and every phone has its own internal bell capacitor, so the third-wire issue doesn't arise.
If I now understand your problem correctly, you have the Maplin adapter plugged into your master jack and an extension connected from the BT outlet on that adapter to a jack upstairs. And a phone connected to that upstairs jack is not giving your dialtone, correct?
If that's the case, then plug the phone directly into the BT jack on the adapter downstairs and check that it works. If it does, then you have a problem in your extension wiring.
Just to clarify this, RJ14 does NOT refer to the fact that there might be four connections in the jack itself. RJ14 is specifically a designation used (in the U.S.) to specify TWO phone lines to the jack.
Ain't that the truth!
Paul
[TELECOM Digest Editor's Note: Well, if you can extract the wires from a phone cord you will find a little tiny strip of metal on the end; this little strip of metal can be _carefully_ inserted into the correct hole on the other half of the socket, and then bent over to make it stay in place. It is quite a hassle, but can be done if no other way is possible. PAT]