Had the same problem a couple of weeks ago, 3 emails to BT and 3 phone calls from India later and all appears to be fine.
My speed dropped from 6 meg to 0.2 meg overnight for no apparent reason about a week after upgrading to Option 3. Couldn't explain it, as nothing had changed at my end, gave them the benefit of doubt and was patient. They told me there was a fault on the line (hard to believe) as I live in a brand new house 500 yards from exchange but they came good and my speed is now back up to an average of 5.5 meg and happy again.
I was sceptical of what they were telling me on the phone but it was resolved in just over a week. Give them a chance to put things right, if you have no joy after about 2 weeks then start to rant a bit. People tend to give Indian call centres a hard time but I will give them their due, they seem to have resolved my speed issue, remains to be seen for how long. They were very quick to reply to emails and phoned me on my mobile at work and at home in an attempt to get my issue resolved.

Speed back to normal after a quick phone call to BT & i had an e-mail from BT Care.
speed back to 6230.21Kbps.
Cant knock BT for doing what they say. They told me that they will check line and do some more tests and came back to me the next day to say all sorted out.
I was also told not to use the re-set button on my Home Hub but to switch off wait for about 15/20 seconds and switch back on again and that might just get the speed back.
Thanks BT