20Mb drop in speed on Fibre

[#1]

Hi
I has my fibre installed & activated a the start of this month, and the speeds I were syncing was the full 80/20.  However, as of yesterday (1st July 2015) at around 8:30am, my line resynced, and the download speed has reduced by approx. 20mb.
Before the drop, my SNR was 5db, with Attenuation at 17db and Output Power at 13db.  My SNR has now increased to 6db, with the other two remaining the same.
I'm unsure as to why the speed has reduced so much?  Nothing has changed at my side to affect this, the only thing I could think is perhaps an OpenReach engineer has been working on the cab at that time & something has been nudged, or some other issue perhaps?
I have tried restarting my modem, but this caused it to sync at 59mb, so dropped by 1 more.  As a note, I can't see this being DLM causing it?  As the line has been totally solid since moving to Fibre, the connection hasn't dropped once (other than when I manually powered off the modem a while back to relocate some cables)
I've attached graphs showing the drop in speed, as I monitor my connection quite closely.
Cheers
Paul

[#2]

I've run a line test and I can't see any obvious issue. It might be worth raising it as a fault, but I'll point out your post to the appropriate team tomorrow.

[#3]

Good Morning,
I've picked this up first thing and from initial testing it appears that your line may be affected by cross talk. 
I'll create a support ticket for you on your account and raise this as a fault to our supplier (BT Wholesale).  They may suggest an engineer, but bear in mind that we may not be able to improve line rates due to the potential cross talk detected.
Impacted estimates are 52Mb upwards according the Broadband Availability Checker at www.dslchecker.bt.com