I'm also one that has such letters, and despite configuring my outer edge firewall to ensure daytime usage never rises over a 1/4 of the maximum speed, I got the second letter. Somewhat annoying as I felt I was complying with the text of the letter. I've essentially switched off all usage during the day because as others have pointed out I'm too scared that I will now be disconnected for breaching the unknown limits. The another slightly annoying thing is there doesn't appear to be any mechanism to "reset" your "bad-boy" count, so that sword is now forever hanging over your neck.
On the other hand, its still quick at night, though now we limit iplayer to later on in the evening which is inconvenient at times. I've also shelved plans to sign up with their TV service - in effect they have lost +£50 per month they could have got and I will consider my options seriously when my renewal comes. I don't know any other business that could be so vague with their rules - imagine if the roads were like that, the speed limit changed daily and there were no signs telling you what the maximum was
