Regarding your original question:
Call sky, (do it during the day, about 10am when your line should be at it's best), and ask them to "reprofile the line, it seems slow".
They should then mess about with the profile and set your maximum sync rate accordingly. Then ask them to "turn off DLM". If they don't understand that, as to speak to tier 2 or the CST.
This will ensure your line profile won't go to hell if there's 1 bad night of weather.
Still do the faceplate though. From your picture, I'm betting some extensions have been wired to the reverse of the incoming linebox, although 1 is definitely running off the faceplate of the current master socket.
This what I'd do:
Take off everything, so you can see the incoming line.
Connect the incoming line to A and B on the reverse of the back part of the linebox (doesn't matter which way round)
Now poke all the extension wiring through the gap in the linebox
Screw the back bit of the linebox to the wall
Punch down any extensions to 2 and 5 on the back of the filtered faceplate ( make sure these are the right way around - best idea is make a note of their location before you start )
Screw on the filtered faceplate
Test a phone at the master socket
Test a phone at an extension socket
Reboot ADSL modem
Then ring Sky and get them to tweak the line as above
FYI: that clear round block is what's known as a "gel crimp". It's used to joint two or more wires for various reasons. It could be that all your extensions are running to the master socket, in a star wiring arrangement, so the block is simply taking all the feeds from the white extension cabling and outputting it via 2 wires, so you only have 1 pair connected to the faceplate.
Gel crimps are low loss and contain a conductive gel that prevents moisture penetration, as they're usually used outdoors to bridge a broken wire etc.