I need a soliciter for a water bill liabilty argument

We are being chased by debt collectors for a Thames Water Bill in my partner's name. The lease agreement on the property explicitly states water is included in the rent payments. All rent is fully paid on time and the lease agreement finished about a year ago. The lettings agent and or landlord are being unresponsive in paying the bill or having the water account transferred to their name to take liability. The lettings agent has after a month now accepted liability but has not done anything about it. We need to threaten legal action and potentially follow through with it if they still do not not take any action.
This issue has caused a lot of stress and anxiety.
We are scared that the Bill casue issues to our credit rating.