I would like to comment on my exchange as i cannot get a direct answer from BT which is very frustrating.
Ok so for 18 months BT rated my line spped at 5-5.5mb on the BT checker. Now all of a sudden its rated at 3mb and also te epected adsl2+ is 3mb. I find this odd as I sync at 5400 - 6200 with download sppeds of 5mb. Sounds great except that i get very slow speeds in the evening.
So whay a drop from 5mb to 3mb on the checker?
Aso i find it hurrendous that bt can change the DSL2+ upgrade date from October 2010 to Dec 2010 then March 2011 and now july 2011
What john46 says about the re-synch is correct. As for the slow speed in the evenings - this is almost certainly due to contention at your exchange. The size of the internet 'pipe' is limited and the more people are using it the less each one of those users will get. Evenings do tend to be busy!