I check all my bills from top to bottom!
(Always have & I always will.)
It is as much your fault as NTL's. As they did provide you with these bills you do have a responsability to check that they are correct.
(and as in the terms & conditions it clealy states you must inform them of any services you are getting which you are not being billed for.)
However on the other hand NTL have more of a responsability as it is their fault the problem occured in the first place!
Personally I would despute the full back dated bills and request some type of compensation for all the hasle they have caused!
I am no fan of NTL as anyone will tell you from my posts on this forum but in this case I don't think it is all their fault. However glad to see they were willing to compromise with you over the duplicate bills!
legally they did not have too!