40mbp is the SYNC speed...
the same as adsl, just because you sync at that speed doesnt mean youll get anywhere near..
I have a 40/10 FTTC connection, and can download at 4.5MB/s
This is due to be getting upgraded to 80/20 next week, so that should theortically double, as im pretty much next to the cabinet.
Also, what you have to remember, when using the speedtests such as speedtest.net, your relying on the upload of the serer, and how many people are using it
It might not work straitght away, but when it does, use
http://www.speedtester.bt.com/