Broadband suddenly dropped to below 500k

[#1]

I'm raising this here because a bunch of wierd stuff popped up in quick succession this morning including my connection becoming unusable for a short time.
It started some time before 6am when my sync rate appears to have dropped to just below 500k. The first sign was that video became unusable. After this I tried resetting the router a few times which led to some LAN side problems that made me suspect the router, at which point I shut down everything and left it.
Now I've restarted everything, I find that:
My sync rate has dropped to 4.48M and this value seems suspiciously constant. I should have at least 6M, its been higher.
BT reports my IP profile at 1M
speedtest.net reports my downstream at about 300k
I'm considering my options, I do have an alternative router but as it is non-wireless the test will be somewhat inconvenient to say the least.

[#2]

Firstly, don't reboot/resync your router more than once a day unless it's an emergency as it may make things worse.
BT's equipment has seen a problem and knocked your IP profile back; it takes several days of stability to recover. 
Second, can you post here some detailed stats from your router (including speeds and attenuation) and the results of the BT Speed Test.
Thirdly, please describe how your router is connected and any phone wiring, extensions and other equipment including location of filters.

[#3]

Thanks
First thing before anything else is I''m hearing crackle on the line. The only thing stopping me calling BT voice faults ASAP is currently I only hear the crackle when the router is connected, so I haven't managed to separate the issue.

Hardware:
Billion 7500GL, firmware 5.52e. Available alternative: Draytek Vigor 2600, but its not wireless which is a big nuisance.
Wiring:
Central NTE5 with faceplate splitter. Several filtered extensions wired in cheap 4 way wire, one computer room extension wired in 3 pair with the unfiltered DSL  circuit on the green pair (this has worked well for at least 2 years).
Stats to follow, I'm posting this while it holds.
Oh and its useless me trying to avoid resync currently, the connection is dropping by itself when there's a bad burst.

[#4]

More information: the Billion isn't great for line stats unless you run syslog (I don't).
Parameters
Downstream Upstream
SNR Margin 17.5 dB 20 dB 
Line Attenuation 21.5 dB  16.0 dB 
CRC Errors 2  0 
Latency Interleave  Interleave 
Upstream 448000
Downstream 4256000

[#5]

Assuming you have a filtered faceplate the first thing to check is if it has gone bad.
Is it possible to try  standard filters  and see it that fixes the problem of noise on the line when the router is connected

[#6]

Pluged a phone into the test socket and camped on "quiet line test" until I heard it for sure, its a voice fault.
Reported as such.
More detail if I can post it before the line fades again. The crackling didn't disappear this time, instead its now continuous to the point that when I reported it the lady i spoke to could also hear it. That's from the test socket with my splitter and extensions all disconnected.
I'm expecting an engineer tomorrow. Hoping they'll fix it.
Don't know if its possible for Plusnet to reset my line speed after its fixed or do I just have to wait three days? Think I'm supposed to resync once a day so it relearns the higher speed or something?
Look at this:
true
Operational Mode G.Dmt
Annex Type AnnexA
Upstream 96000
Downstream 288000
CO Vendor BCLA
Elapsed Time 0 day 0 hr 10 min 44 sec

[#7]

You have to wait unfortunately.
Resynching isn't necessary

[#8]

OK well the phone line's fixed. Here's the BT speed check result:
Test1 comprises of Best Effort Test:  -provides background information.
    Your DSL connection rate: 4768 kbps(DOWN-STREAM),  448 kbps(UP-STREAM)
    IP profile for your line is - 350 kbps
    Actual IP throughput achieved during the test was - 183 kbps
Actually that sync speed seems slow, looking at my stats...
SNR Margin 17.5 dB 20 dB 
Line Attenuation 20.5 dB  15.0 dB 
CRC Errors 2  0 
Latency Interleave  Interleave 
17.5dB!!! I thought broadband typically ran with less than 10dB. Still it'll be fast enough for my needs once the profile resets.
Anyone know why the test result was so much slower than the IP profile?

[#9]

Odds are that because of the noise problems the default noise margin has jumped to 15dB.
Once things have settled down (a couple of weeks is recommended) raise a ticket asking for your default noise margin to be dropped to 6dB and you should get the maximum speed for maxADSL

[#10]

I'd do a resync in the morning. With that line attenuation you should be doing a lot better even if the target has gone to 15dB.