Mortgage Charges and change of terms

In 2006 I took out a mortgage with a company called Capstone who were then taken over a couple of years later by Preffered Mortgages. I have recently had a few quite high late payment charges from Preffered that I disagree with. I phoned up the company and asked for a copy of the document that I originally signed that says I accept these absurd charges. They have sent me a copy of my original mortgage application as well as a copy of Preffered mortgages terms and conditions that they say is valid from when I took out the original loan.

I don't believe I agreed to these charges and I think it's unfair for them to quote terms and conditions from a company that I never signed an agreement with. Surely I am tied to the original agreement with Capstone that makes no mention of these charges.

Can anyone please advise how I should proceed with this.