ADSL connection keeps dropping

[#1]

That cable will be giving you noise pickup problems and so a reduced noise margin which results in slower speed. It will also be susceptible to picking up bursts of noise which could cause you connection to drop.
Do you know if there was any electrical equipment being used (that isn't normally) when you had all those drops? Or any faulty equipment, fluorescent lights etc etc.
If it's not practical to have the filter and modem/router next to the master socket then you should use a proper twisted pair screened cable which I can give you a link for.
These sockets that aren't currently connected - is there one next to you modem/router perchance? Do you know why they aren't connected?

[#2]

I don't know of anything, but we're in a block of flats so could be something our neighbours are doing I guess. When we moved in about 2 years ago we were getting 6-7 Mbps fairly solidly, it's only recently we've got over 10, so I guess the underlying issue could have been there all along. This evening we're back down to 2.8 db, but no disconnections, the last was over a day ago.
There isn't a socket near the modem, the disconnected extensions were like that when we got here, now you mention it it is a little odd that they'd have been intentionally cut off. Please could you post the link for the twisted pair cable? Cheers!

[#3]

OK, I recommend these cables from ADSL Nation Ltd. They are good quality cables from a company with an excellent reputation and their postage cost of £1.58 for orders under £18 is very reasonable compared to many other companies out there. This is the 10m cable. As it's twisted pair cable, a moderate surplus length really doesn't matter and is preferential to one that is too short putting strain on connections and filter.
You would plug the filter direct into the master socket, and the new cable would plug into the modem side of the filter and the other end into your modem/router. If it isn't convenient to have your phone plugged straight into the filter and located by the master socket, then you can use your existing extension cable  to plug into the phone side of the filter, with the phone on the other end.
When the cable arrives and whilst you have everything disconnected, have a look at the rear of your master socket plate and see what wiring is connected. There should only be two wires connected to the terminals, a Blue/white trace on 2, and a White/blue trace on 5. If there are any other wires connected make a note of the colours and terminals they are connected to and post back the information (take pictures if it helps). PS. don't do this whilst your modem/router is connected as any disturbance to the connection could cause multiple disconnects which could be seen as a dropping connection by the Exchange DLM raising the Target Noise Margin and slowing your speed.
HTH.

[#4]

Ok, new cable ordered, delivered and installed. There's definitely an improvement:
Down:
Snr margin 6.0 db
Attenuation 40db
Bit rate 10689 Kbps
Up:
Snr margin 5.9 db
Attenuation 23db
Bit rate 1128 Kbps
I guess we'll keep fingers crossed that that's enough to mitigate the problem happening again. The help is much appreciated - cheers!

[#5]

Oh yes, that does look very much healthier. It seems that other cable (apart from being the wrong type) was a very crappy one. Even if your problem turned out to be external, you are unlikely to get problems with local interference causing drops and certainly considerably less than you would otherwise have had.