I'm being chased for a debt that's only partly mine, but the company have sort of the wrong name for me - England

I'm liable for some of a water bill debt from a flat I lived in 2 years ago.

I sent them an email copy of a letter from my ex landlord, detailing the dates that I lived at the property, however they won't accept this in order to reduce the debt as they have my name partly wrong . They're chasing me for 2 years worth of debt but I was only there for 6 months

For clarification and without using my real name I'll do an example:

My name : Ms Sarah Jones-Smith

What they have on their account: Mr Sarah Smith

They've asked me to provide a letter again with the name they hold for me vs the letter I showed with my real name, I'm not sure how I can do this considering its not accurate? not to mention my ex landlord has since retired and sold the property, so there's no way for me to contact him

I'd love to tell them to naff off as it's not my name on the debt, however I have quite a bizarre name so I'm worried they'd still pin it on me. I also didn't know they held the wrong name until I sent them my landlord's letter otherwise I would have wished them luck in finding Mr Sarah Smith!!

Is there something I can pull out legally to make them either leave me alone or adjust the debt accordingly?

Thanks in advance!