Southern Water - Assessed Charge Bill - Don't think it's right but they won't help

CurlySue38

Hi everyone,
I've been going around in circles trying to figure out my water bill and whether it's actually correct or not. After a pointless webchat with an agent who wouldn't give me any straight answers and an online call with someone who didn't understand my questions I am trying here. 

We live in what is classed as a maisonette - it's a 2 bed extension on the back of a very large house, that is divided up into 10 flats and bedsits. We have been told that we can't have a meter as it's a shared water supply. I think all of the water charges are included in the the flat/bedsit's rent, but ours isn't. Our bill is created as an Assessed charges bill for a 2 bed property. 

When I look at the 'How is my water bill calculated' page that the Southern Water website says show the charges for this year, they don't match my bill. If you use the filter on that page to show charges, there's one table for metered properties that shows standing charges and unit costs for metered properties. There's another table for Assessed Charges (which is just one amount for water and one for wastewater) and then a final one for Unmeasured. 

I tried asking if the assessed charges in the table include the standing charges, as there isn't any info on the table about them but could not get an answer. 

So, I looked further, and downloaded the detailed PDF called Southern Water Household Charges Scheme 2025-26 which is at the foot of that page. On Pg19 and 20 it seems to imply that all charges combine into 1 including the standing charges for water, waste, standing and highway but I am not sure if I am correct about that. 
In a large table in Part 10 on pgs 56 & 57, it shows standing charges for water of £38.88 (ours is £34.21 which is what is in the table as the cost for properties where charges aren't calculated based on bedrooms) and a charge for water of £286.31 (our bill for water is £330.36) and then for wastewater (sewerage) the table shows a,b,c standing charges that add up the same as what's on our bill £100.20 and £364.32 for the measured assessed charge (but our bill is £421.13 for the assessed charge).

There is potentially about £100 difference in the bill, and assuming it was this messed up last year as well, more that we may have overpaid. Considering our bill has gone up from £552 to £886 this year, I want to make sure they aren't taking more than they should get but I don't know how to get any further with it!