Took a contract out with BT in December, the problem is at night and peak times its unusable.

I have a profile of 7.15mb and in the mornings I get 7mb. At night however this is the results.

Download speedachieved during the test was - 0.57 Mbps
For your connection, the acceptable range of speeds is 0.6 Mbps-7.15 Mbps.
Additional Information:
Your DSL Connection Rate :8.13 Mbps(DOWN-STREAM), 0.45 Mbps(UP-STREAM)
IP Profile for your line is - 7.15 Mbps

Thats every night, its unusable. I came from EE and with EE even at peak times 4-5mb was my minimum speed.

you can if its below the estimates on bt wholesale adsl checker

"The upload/download speeds that we quote when you order the service is an estimate only. Once your speeds have settled (usually 3 days after activation), if you are always getting speeds lower than the estimated range that we quoted, you can contact us any time within the first 3 months after you ordered the service and we will try to improve the speeds. If, despite any improvements we try to make, the speeds you get are less than the "minimum guaranteed access line speeds" for your service (go to www.bt.com/yourlinespeed for more details) you will be entitled to cancel the service without paying a charge for ending it early – provided you have followed our instructions in trying to resolve the problem."

have a look here