Thanks. I just realised I the speed test you gave is the same one as I used yesterday. I followed a link on another forum. The results seem to contradict what I got tonight. Something don't seem right.
I took a screen grab at the time
http://s6.postimage.org/s4ll2pnk1/17_12_12_BT_speed_...
Yes - your downstream sync speed has reduced since this test was performed. Here the sync speed is approximately 51Mbps.
That *might* be expected behaviour, due to DLM intervention and the addition of interleaving + error correction.
Can you tell us, with a fair amount of precision, what date & time your new 80/20 package was activated (by BT, not Plusnet), and what date/time both the first and most recent speedtests were done?
I'd recommend that you keep an eye on the downstream profile value, and see if it changes any more. If it does, and changes often, it could be a sign of problems with your line.