Water Rates for flats within one building

Other

Hi, own a building consisting of one retail unit, and 3 self contained flats. There is only one water feed and one water meter for the whole building. I (my company) pay one water supply charge (metered) and one drainage/sewerage charge. For ease I have always factored in a small contribution to the water as part of the rent. I am now being told by United Utilities I am not allowed to do this and even though I will still pay the water for the tenants I will now be given the following bills: One metered water supply and 4 separate drainage/highways charges for the 3 flats and retail unit. The retail part being on a commercial tarif. These 4 separate drainage/highways charges (even on the minimum band) add up to far more than the building as a whole. Therefore more income for the water company. I will therefore being paying to drain more water than the building receives. They are saying it is 'not allowed' for my company, that owns the entire building, to just pay one bill for water in and one bill for water out. Is this correct? It seem sunfair? They make reference to the 'Water Industry Act 1991' but I can't locate where it says I must pay separate bills? I am more than willing to pay but I want to be treated fairly.