Not wishing to **** on anyones Chips here but surely if it was that easy to switch to a Business Package for a one off fee and a slight increase in price surely NTL as shrewd as they are would have covered all bases on this one.
The easiest way for to detect this, like many other companies do is when you try and sign up for a Business deal as a residential customer the first thing they are going to ask for is your VAT Reg Number.
Im guessing it is not mentioned in the emails posted but the disclaimer will no doubt cover that.