Aha! Right, I've done a bit of image-searching, and reapprised myself of what the current socket looks like, which is this:
(Old BT copper connection at top - master connection to legacy distributed house wiring at bottom left)
Having not prised one of these open before, I did not realise that they look like this on the inside (this image pinched):
So, will using the male-to-male cable (which will have to be a long one!) to connect the back of the SH2 to that internal socket (circled red) using a male-to-male cable, connect the house's legacy wiring to the SH2's telephony interface? Many thanks for further support!
You need to disconnect the incoming openreach cable from terminals A&B on the back of master socket then ensure extension wiring is connected to terminals 2&5 and then replace faceplate the cable from hub socket now connects to phone socket on front of master
Sorry, I could have explained better.
If your master socket is as shown in the last pic, you need to further undo the two screws & the A/B connections will be on the back of that. The faceplate should have the extension wiring on 2 & 5 as above, be careful not to pull those out as they're only a push fit.
Once you've disconnected A/B, replace everything. You can then connect the BT431 cable to ANY socket. It doesn't have to be the master & can't go into that test socket you've ringed. The whole purpose of that design is so that when the faceplate is removed, all of the extension wiring is disconnected from the incoming wiring. If you plug in there it won't be going anywhere!