Is it normal for the speed to drop so much after the first day?
Why is the upload speed so slow?
Because the upstream and downstream are weaved together so upstream drops off over distance more than it used to with ADSL2+ and the DLM will have eaten a chunk.
FTTC has only just become available in our area so a lot of people are being connected. Could this have an effect?
Is the target noise margin 6db as with standard ADSL?
The DLM works differently, tends to cap/band speeds rather than work to target margins.
Should I take this up with BT?
Maybe, but if you think line length is closer to 1.5km, then your speeds are what I'd expect once cross talk kicked in.
The fact the maximum rate is close to the data rate suggests you were the first in your bundle, and its the next couple of people in your cable bundle that will have killed your speed I suspect.