Joint tenancy renewal gone wrong

My flat was being rented by 3 people on a joint tenancy. They were paying separately from each other for a year now. Now the year has finished and one of them is gone. The other two told me that they are keeping the flat and they are going to find another person to join them for the new contract. It has been 1 month and they still have not found anyone. Now they are refusing to pay the full amount. What action would you recommend I should take?
I have already told them that it is a joint tenancy and that each one of them is guarantor for the full monthly amount but they still refuse to pay the balance.

Send them a s21 notice. If they have any money sue them if the deposit doesn’t cover it.

As this is an HMO I assume you checked the licence position?


And also make sure you include the third tenant on both notice and when you sue for money.