Sam Knows - White box - does PN understand it? and FTTC slowdowns recently

[#1]

A few months ago I received an email from PlusNet suggesting that I sign up with SamKnows to get a White Box - it communicates with a server somewhere  and logs the performance of the broadband connection. The process provides one with a monthly report showing the performance of the line over the month. In addition one can get performance data at any time.  
Recently I have been seeing quite a significant slowdown in FTTC performance so I sent a copy of the download performance report in a Ticket/Question to PN. The response from PN ignored the Samknows information and asked me to try the BT line test.  In fact there was no reaction to the SamKnows report.
Here are the download speed reports:
For the year to date

The report shows the variation in download speed and specifically the major speed reductions during the last week which I was asking PN about.  
What causes these slow downs, is it a failure in the BT network etc.  Is it PN 's fault - and can they be fixed so that the slowdowns do not occur.
And for the record - no WiFi signal is used connecting the SanKnows box to the BT network.

[#2]

That doesn't look good at all. However, PN need you to use the BT line test facility because BT insist on receiving that data if they are to visit your home in order to fix a possible fault.

[#3]

The download speed is now back to almost normal, using the BT test will only give a snapshot at a given point of time - the SamKnows graph gives a history of the performance that enables investigators to compare it to other  evednts.
The recommended BT speed test web site is treated as a suspect web site by OpenDNS so yet more hassle to comply with to get any information.