Seeing as so many people are reporting the problem on the same exchange, it would seem exchange related.
I would point this out to BT Support, if you haven't already.
Colindale is one of the core nodes in the 21CN network and is always being upgraded/undergoing maintenance. Hopefully, the fix will be quick one (I imagine the VLANs are running hot).
I never managed to run that test. I followed all the steps described on their page but the test never runs. In my case it actually failes at resolving the name. I checked the forum here and I can see that I am not the only one that cannot run that test at all.
So, I had a chat with BT support. They did the same test and could not find any issue on the line. As previously they will be monitoring my line again and then they are going to call me back. It is frustrating because it is third time that I think nothing is going to be done.
Anyway, I run traceroute to see how the packets are going from my hub. Below is what I got
1 192.168.1.254 (192.168.1.254) 4.340 ms 4.118 ms 4.040 ms
2 172.16.11.130 (172.16.11.130) 84.999 ms 13.126 ms 12.474 ms
3 * * *
4 220.127.116.11 (18.104.22.168) 64.860 ms 69.915 ms 43.340 ms
5 22.214.171.124 (126.96.36.199) 19.061 ms 17.592 ms 13.536 ms
6 188.8.131.52 (184.108.40.206) 14.918 ms 18.842 ms 32.188 ms
7 220.127.116.11 (18.104.22.168) 16.178 ms 41.206 ms 49.241 ms
8 acc1-10gige-10-3-0.sf.21cn-ipp.bt.net (22.214.171.124) 47.429 ms
acc1-xe-5-0-3.sf.21cn-ipp.bt.net (126.96.36.199) 97.903 ms
acc1-xe-0-3-1.sf.21cn-ipp.bt.net (188.8.131.52) 13.070 ms
9 core2-te0-4-0-2.ilford.ukcore.bt.net (184.108.40.206) 35.067 ms
core2-te-0-13-0-11.ilford.ukcore.bt.net (220.127.116.11) 88.037 ms
core1-te0-0-0-4.ealing.ukcore.bt.net (18.104.22.168) 34.342 ms
10 peer1-xe0-1-0.telehouse.ukcore.bt.net (22.214.171.124) 42.703 ms 24.572 ms 28.841 ms
11 126.96.36.199 (188.8.131.52) 21.200 ms 36.564 ms 68.432 ms
12 184.108.40.206 (220.127.116.11) 25.257 ms * *
13 18.104.22.168 (22.214.171.124) 79.262 ms 70.894 ms 53.341 ms
14 lhr08s01-in-f31.1e100.net (126.96.36.199) 80.954 ms 30.267 ms 55.554 ms
Anyone who can interpret this to see whether there might be something suspiscious here?
Perhaps this will help to explain my problem. Just noticed that just after midnight the speed goes back to normal for some time. I did manage to get download speed over 50Mbps.
I had exactly the same issue with my previous provider some months ago. I obviously contacted the support and reported the stagnation on my line during certain periods. They obviously had to follow ther script and get all the checks done. Engineer visit has been offered to my house but there is no point as it has nothing to do on my end. The problem has been later fixed (few weeks after) when I constantly kept pushing the support and eventually contacted the CEO complaints department. They haven't explained what the problem was but I can almost guarantee that it was directly connected to traffic managment.
Make sure to have patience, wait and speak with people that have something to say in the support department and don't waste your time on level 1 support.