Have you served a Section 8 notice yet?
Serving the notice properly
Use the current prescribed Form 3A from GOV.UK; older forms are invalid for private tenancies. Fill it in fully: every tenant's name, the full wording of each ground, and the earliest date you can go to court.
Serve it on every tenant by a method the tenancy allows and keep proof of how and when. A Section 8 notice can be used to start court proceedings for 12 months after it's served.
Mistakes on the notice are one of the most common reasons claims fail.