House mate refusing to pay bills

I have been living in the house I am in for 3 years, i live with three people x, y and z. Z has lived with me for a year and a half and x moved in last year oct, and y moved in, in nov. X pays 350 (they are letting the small spare room) The other three of us pay 680 each, and I sort the bills wifi and council tax. X’s 350 was supposed to cover all the bills, but it ended up only covering half. On move in Y agreed on 750 - 800 max per month, then X decided to stay indefinitely, so since nov I have been paying for everything and Z is fully up to date with all the payments and has paid me a 4th every month.

Y is now planning to move out and is refusing to pay any of the bills because they say that the 3rd of the 350 is his and that covers the bills for the amount of time he spends in the house, which is something that is new and hasn’t been discussed or agreed upon. X has left the house this month so hasn’t paid rent which is fine as it they aren’t there they aren’t using any and they aren’t on the contract.

X and Y have not discussed amounts or anything to do with the money,

If anyone can give me any advice, that would be great ahha!