Hence why they're currently busy trying to write some software which will show you, in real-time, exactly how much you have used. Until they get that running, feel free to go over your limit every single month, by how ever much you like and ignore the letters they send you. I have about a years worth of letters now and still no overcharges.
If you go over occasionally, they don't usually charge you any extra. They only start charging when you start exceeding the limit every Month.
But I do hope that when NTL up the speeds to 10MB, they increase the caps accordingly.