Sudden increase in Fibre Speed. What gives?

[#1]

I'm on the 80/20 FTTC unlimited service, and although originally on about 67Mbps, from about Sept 2013 speeds dropped to about 50Mbps and have stayed there ever since.
While working from home today I was getting lag issues connecting to our work vpn.  It's most likely something to do with work, but during the diagnosis I did a speed test to rule out my broadband connection.

What has happened?  Even the BT Infinity speed checker says I'll get between 64 and 80.  This literally seems to have happened in the last hour, since a speed test at about 2pm gave me the usual 48-50 (and all my tests before that).

[#2]

http://forum.kitz.co.uk/index.php/topic,14867.0.html and http://forum.kitz.co.uk/index.php/topic,15099.0.html

Quote

VDSL retransmission is now being deployed at a rate of around 45,000 circuits per day

BT rolling out G.INP/PhyR and changes to DLM.
G.INP reduced the effects of crosstalk
Ian

[#3]

Well now.  Is there any way I can find out if that has happened in my area?

[#4]

Have you got a hg612 OR modem...they can easily be unlocked to see the stats, and see if you are on a line with g.inp.
http://huaweihg612hacking.files.wordpress.com/2011/11/hg612_unlock_instructions_v1-3.pdf