FFTC minimum speed guaranteed ?

[#1]

I have been on FTTC for a number on months now and overall have been very pleased will the result.
More recently, I have seen the download speed drop, particularly during evening periods, from 30Mbps  to 20, 12 or even 3Mbps which is worse than it was on ASDL!
As the FTTC products are being launched, is there a minimum download speed as part of the T&Cs or is it on the basis that this is a function of the network infrastructure and so users need to accept the service as it is?
Will the decrease in performance at peak time be seen as a fault in the hope that more capacity is provided on the network?
Any comment from the Plusnet guys?
Thanks,
Graham

[#2]

Are you on extra or pro and are these download speeds the result of a speed test with AV turned OFF? ( I was getting rediculous results between 0.3Mbps - 40Mbps+ till I realised my AV was affecting it ). Its worth noting that they cant  guarantee any minimum speed, such is the nature of the internet, they can offer a guidline but if you're not on pro then traffic management may account for your speed drops during peak times.
It could also be as you said down to the infrastructure and more people have been connected to your FTTC cab. I believe BT are providing more capacity as they role out FTTC but I dont have the foggiest how this works!
If you are on pro then < 20Mbps does sound a little extreme. I'm on ADSL2+ extra till tuesday (getting FTTC) and have a constant connection speed of 5.3Mbps shown by speed tests so am very interested in your situation.

[#3]

I guess for a drop in speed to be classed as a fault, you need to be able to classify where the drop in speed happens.
Does your BT Speedtester results include an indication of what your current sync rate is, or what the IP profile is?
Those should be able to quantify behaviour of your line, and perhaps the cabinet/exchange.

[#4]

Tests on the line show your line has recorded a min of 30mbps sync, max of 35 and averages around 34.
Can you perform a BT Speedtest as we need further details to investigate the profile of 13000 being assigned.

[#5]

Thank Phil.
The profile on Plusnet shows the expected 20000. 
BT speedtest taken this evening shows the following:
Download speed achieved during the test was - 26414 Kbps
For your connection, the acceptable range of speeds is 12000-35469 Kbps .
Additional Information:
IP Profile for your line is -35469 Kbps
Upload Speed
7983 Kbps
0 Kbps 10000 Kbps
Max Achievable Speed
>Upload speed achieved during the test was - 7983 Kbps
Additional Information:
Upstream Rate IP profile on your line is - 10000 Kbps

Although not bad, when first installed it was around 32-35MB.