I was away abroad when I posted my original question (which I'm glad to say has been kindly answered by MrSaffron and RobertoS), but now that I'm back I thought I would post up some photos of my set up anyway.

Here is the junction box located in the coal cellar (Ground floor) where the black drop line enters the premises. The two white cable are spliced into the black one. One of the white cables may well be redundant, I'm not sure.

https://www.dropbox.com/s/kjk7v2dkhjv102v/IMG_0190.j...
https://www.dropbox.com/s/m025b02b5y76cdo/IMG_0191.j...

Here is what I think is the actual master socket, which I found under the floorboards in the eaves (1st floor). It appears to be, as RobertoS guessed, an LJ2/1A:

https://www.dropbox.com/s/ttqvy8nntchqco8/IMG_0194.j...
https://www.dropbox.com/s/lf65f81b8597uq1/IMG_0195.j...

And here is the extension (LJ4/1A - masquerading as a master socket?) (2nd floor) that I am currently running my ADSL gear off. The second line is redundant.

https://www.dropbox.com/s/cnia1de57yy8br4/IMG_0192.j...
https://www.dropbox.com/s/aj2l641y2v5m08b/IMG_0193.j...

Hopefully the Openreach engineer booked for next Friday to do the FTTC install can simplify all this. All I want is an FTTC master socket that I can plug the modem/router into as well as run a DECT base station off.

Going by the pics should be easy to bypass the old LJ2/1A ( just crip the wires up ) and make the LJ4/1A you are using into a NTE5 . Ps i am taking that you only use the one point ?

At the moment I'm using the LJ2/1A for the DECT base station, mainly because its more central. The ADSL modem plugs into the LJ4/1A upstairs in my office.

I'm thinking I might ask the Openreach engineer to site the new FTTC modem/router where the LJ2/1A is, again because it is more central and will give me Wifi reach into the garden, where the signal is currently sketchy. Presumably that would require them to install a new FTTC master socket and faceplate to replace the LJ2/1A? If they manage to retain the current configuration, I could then also plug a POTS phone into the LJ4/1A extension upstairs.

Also, I've ordered a new Zyxel VMG8324-B10A modem/router from my ISP for the various business features. Would you advise plugging this into the Openreach supplied modem and using it just as a router, or would I get better results if I used it instead of the supplied modem altogether?

If you do that, the LJ4/1A needs de-mastering or better still replacing with an LJ2/3A.

You can try both configurations you describe and see which gives you the best. Which ISP will you be using? If it's Plusnet you need at least 24 hours to test each setup.

My ISP is Eclipse.

I expect any change in performance would be instant then, but can't guarantee it. The only thing that guarantees instant discovery of a better connection is the line stats from the modem or modem/router.