Speed difference after moving from BT

[#1]

Hi,
About a month ago I was using an unlimited BT ADSL packaged on the very same line I'm using for plusnet now, and I had been getting a consistent 13mbit/sec connection. After moving to plusnet it's dropped down to 8mbit/sec. I would've thought that given that everything else is the same that I'd have gotten more or less the same speed as before after changing.
Is there anything I (or anyone else) can do to improve this at all?
Thanks

[#2]

Hi antireality,
I've just placed an order to change the product you were provisioned on. You should see an improvement within the next 24 hours.