automatically within a very few days.
Or not, if you're unlucky.....
Keep an eye on it, DLM does NOT move quickly. And run the BT Speed Tester to check your IP Profile every day or so.
The IP Profile should move up "soon" but the target SNRM, and hence connection speed will take a lot longer.
If it sticks totally contact your ISP, explain that BT have totally rewired EXTERNALLY and INTERNALLY and request a full retrain.
Edit: just seen you're on a fixed package. DLM does not apply in quite the same way until the cap is removed.
I'd ask for the cap to be removed AND a full retrain, once it's settled down.
You should easily get 8mbps, or even 16 if on ADSL2+!