Flatmate won't tell me how much to pay for bills?

I live with two other people, we're all in third year. One of the other tenants and I both send the monthly rent money to the lead tenant who has then sends it to the landlord. Our bills aren't included in our rent so every month we ask the lead tenant how much we owe, they say they'll check later and then never get back to us despite us pestering them about it. They've made jokes before about surprising us with a big charge at random once they "get around" to checking and it's making me really worried.

I'm really regretting agreeing to this person being the lead tenant and I'm really unsure of what to do.