Re: And what you neglected to mention:
@AndrueC
when we talk about Congestion this tends to be the speed you get
the modem will still get 5mb/s or 70mb/s (what ever you get as an Normal lock on your modem is, peak or not peak time does not normal affect this at all) but its the speed at the other end of the phone cable backhaul at the exchange is over utilised so BT QOS is limiting speed so that packet loss does not happen (i have not seen this happen on VDSL yet and even if it does its only be tech people that notice it, eve VDSL can provide 10mb/s that is enough for most)
Congestion on an BT line is more likey to happen on the smaller exchanges (below 2000ish) more so if its not even 21CN yet, there are 2 of them around where i am, ADSL Lock 7700, real download speed around 6pm 0.5-2mb (norm the lower number) drive me nuts when i go around that area later on in the day needing to download stuff as it takes all day (at least BT QOS is working on the exchange to handle the limited backhaul as there is norm no packet loss, UNLIKE Virgin cable where it drops packets when upstream is saturated)
the bigger ones norm have the larger backhaul and have 21cn as well so more fibre there to use and turn on if needed, so speed is norm not an issue on large exchanges or any that have 21CN, if your getting FTTC where you are then the Main exchange will have large pipe already and will be 21CN enabled
the Lock speed for ADSL or VDSL should not change under norm use (peak time or not) the more users that are connected to the "exchange/VDSL cab" can norm lowers the speed an little but not much unless an BT man touches your phone cable in the cab and knocks it lose
normal the modem/exchange will lock the speed an little lower then the best rate you can get (sky lock it the download speed to 500kbs lower then best speed it can get, as to why some times users end up with silly 0.02mb locked profile connections some times, sky its just quick call to reset it, other providers i prefer to kill my self some times when all i am asking them to do is reset the line profile)