Thanks very much guys 
I can't remember which one of the 3 sockets is the master but the original incoming cabling is terminated at another junction box and then the 3 sockets are starred off from there.
The outside wiring runs along the house at 1st floor height and it'd be perfect to run this in to the 1st floor landing for a new master. OR eng said the existing cabling is likely to be better quality than their new copper plated steel stuff.
To summarise
Existing phone line really isn't well
I'd like a new master socket at a better location
I don't really want to pay for a new / replacement line from the pole
I'm recording quiet line tests when the line goes bad
So can I get this fixed by chasing BT to repair the line as per gt94sss2 and then sweet talking the OR eng on the day?
(As it was near Xmas I suspect the prev eng wanted an easy day on the original visit)
P.S the nearest cab matches that checker though I'd have thought they'd have managed something better than using a permanent marker to identify it!
P.P.S Don't you guys have anything better to do on NYE?! 
I suffered similar problems several years ago.
The telephone cabling to my house includes overhead wires, which reach the property via a series of telephone poles. The cabling used to terminate in an old-style BT junction box, screwed inside one of the window frames. The master socket and extension sockets were star wired, with extension wires that ran around the outside of the house. BT had installed this wiring in the 1970s, when the house was built.
High winds and driving rain were causing intermittent problems with my voice and broadband services, so I reported the problem to TalkTalk on many occasions. OR engineers attended repeatedly but each time, the weather had improved so they did not detect any fault. I insisted eventually that they examine all of the overhead wires and junctions between the cabinet and my home.
It transpired that BT's wiring was OK - until it reached my home. However, the original installers had used internal quality cable for the extension sockets, even though the wires were stapled to an outside wall. After 40 years' exposure to sun and rain, the plastic insulation had cracked. So every time it rained, water penetrated the cable and caused a short circuit.
BT Openreach removed the cracked extension wiring and installed a new NTE5 master socket. However, I had to run new wires inside the house to each extension socket. To add insult to injury, TalkTalk charged me for the engineer's visit, because even though the cracked wiring had been installed by BT and was outside the house, the crack was located AFTER the junction box.