Losing speed after being on the trial for months

[#1]

Until recently, I have been running at up to 4.8 Mbps downlink speed and 1 Mbps uplink, which was excellent. However, last week I noticed a drop in the downlink speed to 4300 Kbps and now it is running at 3069. I thought the trial had ended, but that doesn't seem to be the case. My ADSL1 speed was typically 3300 down and 800? up (I'm guessing the latter).
Does the sudden drop in speed mean there's a fault? I tried running the BT speed tester but that reports an error, at the end, of course.
The PlusNet MyBroadband Speed tester gives 2654 Kbps down and 796 up, both of which are lower than the peak result I saved in March, which was 4081 Kbps down and 845 up. Last week it gave 3648 down and 832 up.
I'm running RouterStats to monitor the line. That shows the Rx Sync Speed dropped from 4279 to 3069 just before 9 a.m. this morning. At the time, the noise margin was about 10 dB, implying that wasn't the reason for the drop. It's now about 13 dB but there are some big wiggles in the graph.
Thanks for any suggestions.
David

[#2]

Unfortunately your speed will drop more as a sync rate of 3069 will give you an IP profile of 2500 which normally will give you an actual speed of 2200kbps. BT IP profile drops within 75 minutes after syncing at a lower speed but PN can take several hours to pick this up.  If you go to https://portal.plus.net/my.html?action=data_transfer_speed it will show your current line speed which should reflect BT IP profle. Would not be surprised if it's still showing 3500 which a sync speed of 4279 will give you. Some of the recent drop in speed can be attributed to the bad weather and electrical storms in the last 12 hours.

[#3]

As itsme alludes to, it's quite possible that there's been an increase in SNR following on from the recent weather conditions.
I'd give things a few days and see if your previous speeds return.

[#4]

I was reporting the actual line speeds from the router stats page, as opposed to the profile. It has been in the 3 Mbps range for a couple of weeks but went down completely for a couple of hours this morning. I didn't try using the phone during that time, so I don't know if someone was working on the line (they didn't say). When it came back, it was syncing at 5040, though the profile is still in the 2 Mbps range, of course. Anyway, I have only seen higher speeds very briefly previously, 6 Mbps for 30 minutes before I started the trial, but it stayed at 5040 for an hour until I started using remote control and then disconnected briefly a few times. That's not much good for remote controlling! It's come back up again, still at 5040, so I'll keep an eye on it (or rather, RouterStats will).
Thanks for the suggestions and sorry for the delay replying. I was expecting to get an e-mail notification (is that still not working?).
David
P.S. The noise margin was in the 10-15 dB range while the line speed was in the 3 Mbps range, implying it could be running a bit faster. Now, it's only about 3 dB but quite variable. It's daytime, so I expect the line speed will drop this evening, assuming it lasts that long.