I think I need clarification:
Unless you signed the lease prior to the hand-written part being added, then you are probably liable . If the addition was made after you signed it, then you should have been required to initial the change(and it should have been added to your copy or should have been given a new copy. At any rate, if there is a discrepancy consult your copy, as that should be the binding one. But I'm not sure if that is what you mean.
Are the bills original ones from the utilities company? Or did your landlord draw them up for what he/she determined is your share of the utilities? If yes to the former question, then they should be entirely in your name, and there will be no confusion. If yes to the latter question, I would think you would be entitled to see the original and have a clear understanding of what your share of the total is. Negotiate. How many people are in the other suite/s, it should be based on # of people and not just split in half if there are 3 people in the landlord's suite and you live alone. Do you know what I mean?
Without the original bills as proof, I would think they would not have a case against you, but I'm sure I don't need to caution you that a landlord can make life difficult for a tenant.
Are you in an apartment? A basement suite? Have a bedroom in a shared house arrangement? That would all make a difference and should be considered in the percentage that you are responsible for.
If you have a dispute, check with your Landlord Tenant Act or see if there is a supporting agency. They can favor the landlord or the tenant, depending on how socialistic your area is. Mine unfortunately favored the landlord, but in other Provinces (lucky them) they favored the tenant.
Previous personal experience from years of being a tenant.